if_ovpn: support floating clients

If a client changes its IP address notify userspace of this.

The UDP filtering function supplies the remote IP address, so we check if the
address changed there. If so, we tag the packet with the new address. Once the
packet is decrypted (and as part of that, has had its signature checked) we
can commit to the address change. Take the write lock and notify userspace of
the change.

Reviewed by: markj
MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D51468

(cherry picked from commit 9c52600a5a150117b4396df3b868cf2516e1674c)