Leverage the fact that m_pullup() in an inline function in mbuf.h and let
compiler to do all optimizations.
Mostly mechanical changes, but in few cases surrounding lines are brought
to style(9). In netinet6, a few input functions changed to set the
input/output `struct mbuf **` pointer to NULL at a single place.
Reviewed by: pouria, zlei, gallatin
Differential Revision: https://reviews.freebsd.org/D60689
Gleb Smirnoff
41356c8017