Add required checks for unmapped mbufs in ipdivert and ipfw

Also add an M_ASSERTMAPPED() macro to verify that all mbufs in the chain
are mapped. Use it in ipfw_nat, which operates on a chain returned by
m_megapullup().

PR: 255164
Reviewed by: ae, gallatin
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29838