ipsec: Add support for PMTUD for IPv6 tunnels

Discard and send ICMPv6 Packet Too Big to sender when we try to encapsulate
and forward a packet which total length exceeds the PMTU.
Logic is based on the IPv4 implementation.
Common code was moved to a separate function.

Differential revision: https://reviews.freebsd.org/D31771
Obtained from: Semihalf
Sponsored by: Stormshield