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

[bag_semihalf.com]

Summary

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.

[melifaro]

sys/netipsec/ipsec_output.c

357

Why can't we reuse the previous case?

[bag_semihalf.com]

sys/netipsec/ipsec_output.c

357

In my option this way is clearer what the code does. And I don’t think we should call tcp_maxmtu when we don’t need it, because it searches through the routing table.