xfs: ubsan fixes

[ Upstream commit 22a6c83777ac7c17d6c63891beeeac24cf5da450 ]

Fix some complaints from the UBSAN about signed integer addition overflows.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>