MFC r328876:

MFC r328876:
  Modify ip6_get_prevhdr() to be able use it safely.

  Instead of returning pointer to the previous header, return its offset.
  In frag6_input() use m_copyback() and determined offset to store next
  header instead of accessing to it by pointer and assuming that the memory
  is contiguous.

  In rip6_input() use offset returned by ip6_get_prevhdr() instead of
  calculating it from pointers arithmetic, because IP header can belong
  to another mbuf in the chain.

  Reported by:	Maxime Villard <max at m00nbsd dot net>

git-svn-id: svn://svn.freebsd.org/base/stable/10@329158 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f