btrfs: fix iterator with update error in backref.c

Fix the following error:

fs/btrfs/backref.c:565:1-20: iterator with update on line 577

Fixes: a7ca422('btrfs: use list_for_each_entry* in backref.c')
Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -560,13 +560,13 @@ static int __add_missing_keys(struct btrfs_fs_info *fs_info,
  */
 static void __merge_refs(struct list_head *head, int mode)
 {
-	struct __prelim_ref *ref1;
+	struct __prelim_ref *pos1;
 
-	list_for_each_entry(ref1, head, list) {
-		struct __prelim_ref *ref2 = ref1, *tmp;
+	list_for_each_entry(pos1, head, list) {
+		struct __prelim_ref *pos2 = pos1, *tmp;
 
-		list_for_each_entry_safe_continue(ref2, tmp, head, list) {
-			struct __prelim_ref *xchg;
+		list_for_each_entry_safe_continue(pos2, tmp, head, list) {
+			struct __prelim_ref *xchg, *ref1 = pos1, *ref2 = pos2;
 			struct extent_inode_elem *eie;
 
 			if (!ref_for_same_block(ref1, ref2))