Description of problem:

glusterfs client crashed with the below backtrace while migrating the fds from old graph to the newer graph.

(gdb) bt
#0  0x00007f00bf7a51c6 in dht_fsync (frame=0x7f00c9351250, this=<value optimized out>, fd=0x366242c, datasync=0, 
    xdata=0x0) at dht-inode-read.c:818
#1  0x00007f00bf560359 in wb_fsync (frame=0x7f00c93513a8, this=0x31b9a50, fd=0x366242c, datasync=0, xdata=0x0)
    at write-behind.c:1523
#2  0x00007f00bf34fa9a in ra_fsync (frame=0x7f00c93515ac, this=0x31ba4b0, fd=0x366242c, datasync=0, xdata=0x0)
    at read-ahead.c:628
#3  0x0000003c7e0265cb in default_fsync (frame=0x7f00c93515ac, this=0x31bb000, fd=0x366242c, flags=0, 
    xdata=<value optimized out>) at defaults.c:1795
#4  0x0000003c7e0265cb in default_fsync (frame=0x7f00c93515ac, this=0x31bbc10, fd=0x366242c, flags=0, 
    xdata=<value optimized out>) at defaults.c:1795
#5  0x0000003c7e0265cb in default_fsync (frame=0x7f00c93515ac, this=0x31bc7a0, fd=0x366242c, flags=0, 
    xdata=<value optimized out>) at defaults.c:1795
#6  0x00007f00beb21722 in mdc_fsync (frame=0x7f00c9351d10, this=0x31bd3a0, fd=0x366242c, datasync=0, xdata=0x0)
    at md-cache.c:1654
#7  0x0000003c7e0265cb in default_fsync (frame=0x7f00c9351d10, this=0x31becc0, fd=0x366242c, flags=0, 
    xdata=<value optimized out>) at defaults.c:1795
#8  0x00007f00be6fd915 in io_stats_fsync (frame=0x7f00c9351908, this=0x31bfa40, fd=0x366242c, flags=0, xdata=0x0)
    at io-stats.c:2194
#9  0x0000003c7e0265cb in default_fsync (frame=0x7f00c9351908, this=0x31c0760, fd=0x366242c, flags=0, 
    xdata=<value optimized out>) at defaults.c:1795
#10 0x0000003c7e0684e2 in syncop_fsync (subvol=0x31c0760, fd=0x366242c, dataonly=0) at syncop.c:1921
#11 0x00007f00c2ac43e5 in fuse_migrate_fd (this=0x12ec8e0, basefd=0x366242c, old_subvol=0x31c0760, new_subvol=0x368a540)
    at fuse-bridge.c:4382
#12 0x00007f00c2ac459c in fuse_handle_opened_fds (this=0x12ec8e0, old_subvol=0x31c0760, new_subvol=0x368a540)
    at fuse-bridge.c:4467
#13 0x00007f00c2ac4649 in fuse_graph_switch_task (data=<value optimized out>) at fuse-bridge.c:4518
#14 0x0000003c7e05c222 in synctask_wrap (old_task=<value optimized out>) at syncop.c:333
#15 0x000000304ac438f0 in ?? () from /lib64/libc.so.6
#16 0x0000000000000000 in ?? ()
f 0
#0  0x00007f00bf7a51c6 in dht_fsync (frame=0x7f00c9351250, this=<value optimized out>, fd=0x366242c, datasync=0, 
    xdata=0x0) at dht-inode-read.c:818
818	        STACK_WIND (frame, dht_fsync_cbk, subvol, subvol->fops->fsync,
(gdb) l
813	        local->call_cnt = 1;
814	        local->rebalance.flags = datasync;
815	
816	        subvol = local->cached_subvol;
817	
818	        STACK_WIND (frame, dht_fsync_cbk, subvol, subvol->fops->fsync,
819	                    fd, datasync, xdata);
820	
821	        return 0;
822	
(gdb) p local
$1 = <value optimized out>
(gdb)  p local->cached_subvol 
$1 = (xlator_t *) 0x0
f 6
#6  0x00007f522eaa5dc3 in default_fsync (frame=0x7f521000122c, this=0xfb1280, fd=0x7f5218001dec, flags=0, xdata=0x0)
    at ../../../libglusterfs/src/defaults.c:1795
1795	        STACK_WIND_TAIL (frame, FIRST_CHILD(this),
(gdb) p *fd->inode->fd_ctx
There is no member named fd_ctx.
(gdb) p *this
$2 = {name = 0xfb0ac0 "vol-snapview-client", type = 0xfb1da0 "features/snapview-client", next = 0xfaff00, 
  prev = 0xfb2820, parents = 0xfb3b50, children = 0xfb2760, options = 0xfb1cfc, dlhandle = 0xfb1ea0, 
  fops = 0x7f521fdedc00 <fops>, cbks = 0x7f521fdedee0 <cbks>, dumpops = 0x0, volume_options = {next = 0xfb25f0, 
    prev = 0xfb25f0}, fini = 0x7f521fbea2aa <svc_forget+228>, init = 0x7f521fbe9f69 <svc_flush+729>, 
  reconfigure = 0x7f521fbe9e49 <svc_flush+441>, mem_acct_init = 0x7f521fbe9ef2 <svc_flush+610>, 
  notify = 0x7f521fbea31c <reconfigure+56>, loglevel = GF_LOG_NONE, latencies = {{min = 0, max = 0, total = 0, std = 0, 
      mean = 0, count = 0} <repeats 49 times>}, history = 0x0, ctx = 0xf5f010, graph = 0xf9fad0, itable = 0x0, 
  init_succeeded = 1 '\001', private = 0xfb9eb0, mem_acct = {num_types = 118, rec = 0xfb8c10}, winds = 0, 
  switched = 0 '\000', local_pool = 0xfb9f00, is_autoloaded = _gf_false}
(gdb) p *fd->inode->_ctx
$3 = {{key = 16454272, xl_key = 0xfb1280}, {value1 = 2, ptr1 = 0x2}, {value2 = 0, ptr2 = 0x0}}
(gdb) p *fd->inode->_ctx->xl_key
$4 = {name = 0xfb0ac0 "vol-snapview-client", type = 0xfb1da0 "features/snapview-client", next = 0xfaff00, 
  prev = 0xfb2820, parents = 0xfb3b50, children = 0xfb2760, options = 0xfb1cfc, dlhandle = 0xfb1ea0, 
  fops = 0x7f521fdedc00 <fops>, cbks = 0x7f521fdedee0 <cbks>, dumpops = 0x0, volume_options = {next = 0xfb25f0, 
    prev = 0xfb25f0}, fini = 0x7f521fbea2aa <svc_forget+228>, init = 0x7f521fbe9f69 <svc_flush+729>, 
  reconfigure = 0x7f521fbe9e49 <svc_flush+441>, mem_acct_init = 0x7f521fbe9ef2 <svc_flush+610>, 
  notify = 0x7f521fbea31c <reconfigure+56>, loglevel = GF_LOG_NONE, latencies = {{min = 0, max = 0, total = 0, std = 0, 
      mean = 0, count = 0} <repeats 49 times>}, history = 0x0, ctx = 0xf5f010, graph = 0xf9fad0, itable = 0x0, 
  init_succeeded = 1 '\001', private = 0xfb9eb0, mem_acct = {num_types = 118, rec = 0xfb8c10}, winds = 0, 
  switched = 0 '\000', local_pool = 0xfb9f00, is_autoloaded = _gf_false}
p *fd->inode->_ctx
$5 = {{key = 16454272, xl_key = 0xfb1280}, {value1 = 2, ptr1 = 0x2}, {value2 = 0, ptr2 = 0x0}}



Version-Release number of selected component (if applicable):


How reproducible:

keep doing ls in .snaps directory when USS is enabled and do graph changes (all it has to do is a fd has to be opened and then graph switch has to happen)

Steps to Reproduce:
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2.
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Actual results:


Expected results:


Additional info:

snapview-client does not implement fsync fop.