xfs: fix units conversion error in xfs_bmap_del_extent_delay

The unit conversions in this function do not make sense.  First we
convert a block count to bytes, then divide that bytes value by
rextsize, which is in blocks, to get an rt extent count.  You can't
divide bytes by blocks to get a (possibly multiblock) extent value.

Fortunately nobody uses delalloc on the rt volume so this hasn't
mattered.

Fixes: fa5c836c ("xfs: refactor xfs_bunmapi_cow")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/libxfs/xfs_bmap.c

@@ -4827,7 +4827,7 @@ xfs_bmap_del_extent_delay(
 	ASSERT(got_endoff >= del_endoff);
 
 	if (isrt) {
-		uint64_t rtexts = XFS_FSB_TO_B(mp, del->br_blockcount);
+		uint64_t	rtexts = del->br_blockcount;
 
 		do_div(rtexts, mp->m_sb.sb_rextsize);
 		xfs_mod_frextents(mp, rtexts);