ipsec: If no PMTU in hostcache assume it's equal to link's MTU

If we fail to find to PMTU in hostcache, we assume it's equal
to link's MTU.

This patch prevents packets larger then link's MTU to be dropped
silently if there is no PMTU in hostcache.

Differential revision: https://reviews.freebsd.org/D31770
Obtained from: Semihalf
Sponsored by: Stormshield