gfs2: Change BUG_ON to an assert_withdraw in gfs2_quota_change

Before this patch, gfs2_quota_change() would BUG_ON if the
qa_ref counter was not a positive number. This patch changes it to
be a withdraw instead. That way we can debug things more easily.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/quota.c

@@ -1270,7 +1270,9 @@ void gfs2_quota_change(struct gfs2_inode *ip, s64 change,
 	if (ip->i_diskflags & GFS2_DIF_SYSTEM)
 		return;
 
-	BUG_ON(ip->i_qadata->qa_ref <= 0);
+	if (gfs2_assert_withdraw(sdp, ip->i_qadata &&
+				 ip->i_qadata->qa_ref > 0))
+		return;
 	for (x = 0; x < ip->i_qadata->qa_qd_num; x++) {
 		qd = ip->i_qadata->qa_qd[x];