IP6_EXTHDR_CHECK(): remove the last instances

While r354748 removed almost all IP6_EXTHDR_CHECK() calls, these
are not part of the PULLDOWN_TESTS.
Equally convert these IP6_EXTHDR_CHECK()s here to m_pullup() and remove
the extra check and m_pullup() in tcp_input() under isipv6 given
tcp6_input() has done exactly that pullup already.

MFC after: 8 weeks
Sponsored by: Netflix