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

mainline inclusion
from mainline-5.1-rc1
commit 87c9607d
category: bugfix
bugzilla: 18943
CVE: NA

---------------------------

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: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: yu kuai <yukuai3@huawei.com>
Reviewed-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>