Description of problem:

With https://review.gluster.org/#/c/glusterfs/+/21783/, we have made changes to offload processing upcall notifications to synctask so as not to block epoll threads. However seems like the purpose wasnt fully resolved. 

In "glfs_cbk_upcall_data" -> "synctask_new1" after creating synctask if there is no callback defined, the thread waits on synctask_join till the syncfn is finished. So that way even with those changes, epoll threads are blocked till the upcalls are processed.

Hence the right fix now is to define a callback function for that synctask "glfs_cbk_upcall_syncop" so as to unblock epoll/notify threads completely and the upcall processing can happen in parallel by synctask threads.

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