netinet*: update *mp to pass the proper value back

In ip6_[direct_]input() we are looping over the extension headers
to deal with the next header.  We pass a pointer to an mbuf pointer
to the handling functions.  In certain cases the mbuf can be updated
there and we need to pass the new one back.  That missing in
dest6_input() and route6_input().  In tcp6_input() we should also
update it before we call tcp_input().

The merge is extracted of a larger change in head.

PR:			[[ https://bugs.freebsd.org/240135 | 240135 ]]
Reported by:		prabhakar.lakhera gmail.com