MFC r352511: Improve IPv6 handling over the loopback interface

When processing an incoming IPv6 packet over the loopback interface which
contains Hop-by-Hop options, the mbuf chain is potentially changed in
ip6_hopopts_input(), called by ip6_input_hbh().
This can happen, because of the the use of IP6_EXTHDR_CHECK, which might
call m_pullup().
So provide the updated pointer back to the called of ip6_input_hbh() to
avoid using a freed mbuf chain in`ip6_input()`.

Reviewed by: markj
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D21664