btrfs: ref-verify: fix memory leak in btrfs_ref_tree_mod

There is one error handling path that does not free ref, which may cause
a minor memory leak.

CC: stable@vger.kernel.org # 4.19+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>