xfs: "optimize" buffer item log segment bitmap setting

Optimize the setting of full words of bits in xfs_buf_item_log_segment.
The optimization is purely within the bug triage process.  No functional
changes.

Coverity-id: 1446793
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>