mbuf: add m_free_raw to be used instead of directly calling uma_zfree

The intent is to remove all direct zone_mbuf consumers so that ctor/dtor
from that zone can be reimplemented as wrappers around uma, avoiding an
indirect function call.

Reviewed by: kbowling
Discussed with: gallatin
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30959

(cherry picked from commit 05462babd424124762375dca186802d7355af566)