1. wg_handle_msg_data frees m but the other wg_handle_msg_* just take
a pointer to the mbuf content and not m itself, so free m in those
cases.
2. Can't trivially prove that the pcq is empty by the time
wg_destroy_peer runs pcq_destroy, so let's explicitly purge it
just in case.
3. If wg_send_udp isn't doing udp_send or udp6_output, it still has
to free m in the !INET6 error branch for IPv6 packets.
4. After rumpuser_wg_send_peer or rumpuser_wg_send_user, we still
need to free the mbuf.
5dfb9270fc