Extend m_apply() to support unmapped mbufs.

m_apply() invokes the callback function separately on each segment of
an unmapped mbuf: the TLS header, individual pages, and the TLS
trailer.

Reviewed by: markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30132

(cherry picked from commit 3c7a01d773ac2d128eabb596eed7098f76966cc5)