btrfs: fix root ref counts in error handling in btrfs_get_root_ref

In btrfs_get_root_ref(), when btrfs_insert_fs_root() fails,
btrfs_put_root() can happen for two reasons:

- the root already exists in the tree, in that case it returns the
  reference obtained in btrfs_lookup_fs_root()

- another error so the cleanup is done in the fail label

Calling btrfs_put_root() unconditionally would lead to double decrement
of the root reference possibly freeing it in the second case.
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Fixes: bc44d7c4 ("btrfs: push btrfs_grab_fs_root into btrfs_get_fs_root")
CC: stable@vger.kernel.org # 5.10+
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -1849,9 +1849,10 @@ static struct btrfs_root *btrfs_get_root_ref(struct btrfs_fs_info *fs_info,
 
 	ret = btrfs_insert_fs_root(fs_info, root);
 	if (ret) {
-		btrfs_put_root(root);
-		if (ret == -EEXIST)
+		if (ret == -EEXIST) {
+			btrfs_put_root(root);
 			goto again;
+		}
 		goto fail;
 	}
 	return root;