ipv6: account for jumbo payload option

If a jumbo payload option is added, the length of the mbuf chain is
increased by 8 but the actual hop-by-hop extension header with the
jumbo playload option is only inserted in the packet if there are
other options. Therefore, adjust optlen to reflect the actual size
of IPv6 extension headers including the hop-by-hop extension header
containing the jumbo payload option.

Reported by: syzbot+73fe316271df473230eb@syzkaller.appspotmail.com
Reviewed by: markj, Timo Voelker
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54394