ipfw: Teach ipfw that EtherIP is an upper layer protocol

so that we do not discard EtherIP packets ( over IPv6 network ) when
net.inet6.ip6.fw.deny_unknown_exthdrs is set to 1 ( which is the default
value ).

PR: 227450
Reviewed by: ae, network
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52566