inet: Cleanup on charging memory for newly accepted sockets

If there is no net-memcg associated with the sock, don't bother
calculating its memory usage for charge.

Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
Link: https://lore.kernel.org/r/20230620092712.16217-1-wuyun.abel@bytedance.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>