mbuf: add mbuf information to KASSERTs

Be more consistent about printing the mbuf pointer in KASSERT messages.
This massively helps debugging and we were already doing a good job at
it.

Also replace some handrolled KASSERTs with M_ASSERTPKTHDR() for fewer
copies of the check logic.

In m_align() move the msg into the KASSERT given after it was moved
here in ed6a66ca6c206 the msg is only used in one place.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: glebius, zlei
Differential Revision: https://reviews.freebsd.org/D49817