mbuf zones: take out the trash

The mbuf zones were explicitly specifying the uma trash procedures on
zcreate, conditionally on INVARIANTS, because that used to be necessary
in order to get use-after-free checking for uma zones with non-empty
constructors or destructors. After r355137 uma automatically invokes
the trash constructor and destructor as long as no init and fini are
specified. This now allows the mbuf zones to pass their constructors
and destructors without needing to add on the uma trash procedures
conditionally.

Reviewed by: cem, jhb, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22583