mbuf: add m_dataobj() and m_databuf() bounded accessors

[abhijeetsharma2002_gmail.com]

Summary

m_dataobj(m, t) returns one object of pointer type t or NULL, and
m_databuf() returns the data bounded by the length it stores. Header
only, no KBI change, debug.bounds_dataobj_test and
debug.bounds_databuf_test test them.

Sponsored by: The FreeBSD Foundation

[rpaulo]

sys/kern/kern_bounds_test.c

300

Might be better to actually get true EXTPG mbuf.