Description of problem:
2 replica pure replicate volume. 1 fuse client. Started running ping_pong on the client and, then enable quota, set limit on /, disabled write-behind. While tests were going on enabled client side io-threads. Then brought down a brick and started it, and the brick crashed in __mq_add_new_contribution_node (nice loc->parent was NULL as now parent need not be resolved because of gfid based backend changes and for that crash a bug has been filed). So disabled quota, brought down a brick and started volume force. glusterfs server crashed with the following backtrace.

Core was generated by `/usr/local/sbin/glusterfsd -s localhost --volfile-id mirror.hyperspace.mnt-sda7'.
Program terminated with signal 11, Segmentation fault.
#0  0x00007f061fbdf1d3 in pl_lk (frame=0x7f0626682658, this=0x67b1d0, fd=0x0, cmd=7, flock=0x7f0626355320)
    at ../../../../../xlators/features/locks/src/posix.c:988
988	        pl_inode = pl_inode_get (this, fd->inode);
(gdb) bt
#0  0x00007f061fbdf1d3 in pl_lk (frame=0x7f0626682658, this=0x67b1d0, fd=0x0, cmd=7, flock=0x7f0626355320)
    at ../../../../../xlators/features/locks/src/posix.c:988
#1  0x00007f061f9c4e5b in iot_lk_wrapper (frame=0x7f06266825ac, this=0x67c370, fd=0x0, cmd=7, flock=0x7f0626355320)
    at ../../../../../xlators/performance/io-threads/src/io-threads.c:1114
#2  0x00007f06280ba2cc in call_resume_wind (stub=0x7f06263552d8) at ../../../libglusterfs/src/call-stub.c:2350
#3  0x00007f06280c1278 in call_resume (stub=0x7f06263552d8) at ../../../libglusterfs/src/call-stub.c:3853
#4  0x00007f061f9be7c0 in iot_worker (data=0x686ed0) at ../../../../../xlators/performance/io-threads/src/io-threads.c:138
#5  0x00007f062782ed8c in start_thread (arg=0x7f0624146700) at pthread_create.c:304
#6  0x00007f062757a04d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#7  0x0000000000000000 in ?? ()
(gdb) info thr
  6 Thread 29621  do_sigwait (set=<value optimized out>, sig=0x7f0626353eb8)
    at ../nptl/sysdeps/unix/sysv/linux/../../../../../sysdeps/unix/sysv/linux/sigwait.c:65
  5 Thread 29620  __lll_unlock_wake () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:373
  4 Thread 29622  pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162
  3 Thread 29635  pthread_cond_timedwait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_timedwait.S:216
  2 Thread 29624  0x00007f06278374bd in nanosleep () at ../sysdeps/unix/syscall-template.S:82
* 1 Thread 29634  0x00007f061fbdf1d3 in pl_lk (frame=0x7f0626682658, this=0x67b1d0, fd=0x0, cmd=7, flock=0x7f0626355320)
    at ../../../../../xlators/features/locks/src/posix.c:988
(gdb) f 0
#0  0x00007f061fbdf1d3 in pl_lk (frame=0x7f0626682658, this=0x67b1d0, fd=0x0, cmd=7, flock=0x7f0626355320)
    at ../../../../../xlators/features/locks/src/posix.c:988
988	        pl_inode = pl_inode_get (this, fd->inode);
(gdb) p fd
$1 = (fd_t *) 0x0


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