wg: Fix some mbuf leaks in the input path

[markj]

Summary

• When m_defrag() fails we have to free the mbuf ourselves.
• If the handshake input queue is full, we are responsible for dropping the packet.

[markj]

sys/dev/if_wg/module/if_wg_session.c

1939

How is this queue locked?

[afedorov]

sys/dev/if_wg/module/if_wg_session.c

1908

Hmm, is m_defrag() really needed here? This is a very costly operation due to allocation. Because we only need to get "pkttype", maybe using m_pullup is enough?

[markj]

sys/dev/if_wg/module/if_wg_session.c

1908

That's a good question. wg_handshake() assumes that the entire packet is contiguous, though, so a pullup to get the packet type is not sufficient.