genet: Fix problem with forwarding some TCP/IPv6 packets

TCP/IPv6 packets to be forwarded can be laid out with only the Ethernet
header in the first mbuf, and these packets are lost. There was a
previous hack to pullup ICMPv6 packets with such a layout for the
same reason. Generalize, and pullup any IPv6 packets with only the
Ethernet header in the first mbuf. Possibly this should also include
IPv4, but that situation has not been observed to fail.

PR: 254060
Reported by: denis at h3q.com
MFC after: 3 days