In Christoph's patch to refactor xfs_bmse_merge, the updated rmap code
does more work than it needs to (because map-extent auto-merges
records).  Remove the unnecessary unmap and save ourselves a deferred
op.
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>