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.
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