This fixes buffered (not O_DIRECT) NFS writes in Linux - previously they were
hanging in an infinite loop because COMMIT didn't return the same verifier as
previous WRITEs, and NFS kernel client was infinitely retrying the same writes.
Also this probably allows for correct NFS failover, at least for the same
buffered writes, because NFS clients repeat all write requests until a COMMIT
confirms them.
Previously, multiple parallel writes extending file size through NFS were
racing with each other and triggering deletions of part of the written data
I.e. if you mounted vitastor-nfs and just copied a file into it in MC then
you could end up with only a part of the file actually written
Vitaliy Filippov