mbuf: add m_dataptr() for length-checked access to mbuf data

[abhijeetsharma2002_gmail.com]

Summary

m_dataptr(m, off, len) returns a pointer to len bytes at off in m, or
NULL if m does not hold them, in every build. Under -fbounds-safety the
result is bounded to len. debug.bounds_dataptr_test exercises it.

Sponsored by: The FreeBSD Foundation