if_ovpn: allow peer lookup by vpn4/vpn6 address

[kp]

Summary

Introduce two more RB_TREEs so that we can look up peers by their peer
id (already present) or vpn4 or vpn6 address.
This removes the last linear scan of the peer list.

Sponsored by: Rubicon Communications, LLC ("Netgate")

[zlei]

sys/net/if_ovpn.c

299

memcmp might be overweight here.

306

I'd like to use clang / gcc extension __uint128_t to simplify and make the compare performant, but it seems kernel is not ready for it yet ( not available from sys/_stdint.h ).

Maybe just leave as is or convert to two compare of uint64_t .

1688

Compiler is smart :)

1702

Ditto.

[kp]

Assign IP address directly, rather than memcpy()