Remove redundant ip6_plen initialization.

MFC after:	1 week
Sponsored by:	Yandex LLC


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274465 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Strip IP header only when we act in tunnel mode.

MFC after:	1 week
Sponsored by:	Yandex LLC


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274466 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f