netinet6: Remove PULLDOWN_TESTs.

Remove the KAME introduced PULLDOWN_TESTs which did not even
have a compile-time option in sys/conf to turn them on for a
custom kernel build. They made the code a lot harder to read
or more complicated in a few cases.

Convert the IP6_EXTHDR_CHECK() calls into FreeBSD looking code.
Rather than throwing the packet away if it would not fit the
KAME mbuf expectations, convert the macros to m_pullup() calls.
Do not do any extra manual conditional checks upfront as to
whether the m_len would suffice (*), simply let m_pullup() do
its work (incl. an early check).

Remove extra m_pullup() calls where earlier in the function or
the only caller has already done the pullup.

Discussed with: rwatson (*)
Reviewed by: ae
MFC after: 8 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D22334