xfs: fix inobt magic number check

In commit a6a781a5 ("xfs: have buffer verifier functions
report failing address") the bad magic number return was ported
incorrectly.

Fixes: a6a781a5
Reported-by:  <syzbot+08ab33be0178b76851c8@syzkaller.appspotmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>