pnfs/blocklayout: update last_write_offset atomically with extents

pnfs/blocklayout: update last_write_offset atomically with extents

Block/SCSI layout write completion may add committable extents to the
extent tree before updating the layout's last-written byte under the inode
lock.  If a sync happens before this value is updated, then
prepare_layoutcommit may find and encode these extents which would produce
a LAYOUTCOMMIT request whose encoded extents are larger than the request's
loca_length.

Fix this by using a last-written byte value that is updated atomically with
the extent tree so that commitable extents always match.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>