mbuf: improve KASSERT(9) falure messages in the m_apply()
- Make less ambiguous;
- extend to provide more context for post-mortem.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D43776
MFC after: 2 weeks
(cherry picked from commit a43fb3653b35ab88b72e1277dccc26328363cdb7)