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>

fs/xfs/libxfs/xfs_ialloc_btree.c

@@ -296,7 +296,7 @@ xfs_inobt_verify(
 	case cpu_to_be32(XFS_FIBT_MAGIC):
 		break;
 	default:
-		return NULL;
+		return __this_address;
 	}
 
 	/* level verification */