Btrfs: add missing check for dir != tmp_dir to is_first_ref

We missed that check which resultet in all refs with the same name
being reported as first_ref.

Reported-by: Alex Lyakas <alex.bolshoy.btrfs@gmail.com>
Signed-off-by: Alexander Block <ablock84@googlemail.com>

fs/btrfs/send.c

@@ -1589,7 +1589,7 @@ static int is_first_ref(struct send_ctx *sctx,
 	if (ret < 0)
 		goto out;
 
-	if (name_len != fs_path_len(tmp_name)) {
+	if (dir != tmp_dir || name_len != fs_path_len(tmp_name)) {
 		ret = 0;
 		goto out;
 	}