xfs: fix off-by-one error in rtbitmap cross-reference

Fix an off-by-one error in the realtime bitmap "is used" cross-reference
helper function if the realtime extent size is a single block.
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NBrian Foster <bfoster@redhat.com>

fs/xfs/scrub/rtbitmap.c

@@ -141,9 +141,8 @@ xchk_xref_is_used_rt_space(
 	startext = fsbno;
 	endext = fsbno + len - 1;
 	do_div(startext, sc->mp->m_sb.sb_rextsize);
-	if (do_div(endext, sc->mp->m_sb.sb_rextsize))
-		endext++;
-	extcount = endext - startext;
+	do_div(endext, sc->mp->m_sb.sb_rextsize);
+	extcount = endext - startext + 1;
 	xfs_ilock(sc->mp->m_rbmip, XFS_ILOCK_SHARED | XFS_ILOCK_RTBITMAP);
 	error = xfs_rtalloc_extent_is_free(sc->mp, sc->tp, startext, extcount,
 			&is_free);