btrfs: backref: Don't merge refs which are not for same block.

Old __merge_refs() in backref.c will even merge refs whose root_id are
different, which makes qgroup gives wrong result.

Fix it by checking ref_for_same_block() before any mode specific works.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/backref.c

@@ -509,7 +509,7 @@ static int __add_missing_keys(struct btrfs_fs_info *fs_info,
 }
 
 /*
- * merge two lists of backrefs and adjust counts accordingly
+ * merge backrefs and adjust counts accordingly
  *
  * mode = 1: merge identical keys, if key is set
  *    FIXME: if we add more keys in __add_prelim_ref, we can merge more here.
@@ -537,9 +537,9 @@ static void __merge_refs(struct list_head *head, int mode)
 
 			ref2 = list_entry(pos2, struct __prelim_ref, list);
 
-			if (mode == 1) {
 			if (!ref_for_same_block(ref1, ref2))
 				continue;
+			if (mode == 1) {
 				if (!ref1->parent && ref2->parent) {
 					xchg = ref1;
 					ref1 = ref2;