remove hardcoded assumptions on if_mtu.  leave it to sys/net. makes it possible to take advantage of 9K ethernet.
remove hardcoded assumptions on if_mtu.  leave it to sys/net.
makes it possible to take advantage of 9K ethernet.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars