Add a UMA zone flag to disable the use of buckets.

This allows the creation of zones which don't do any caching in front of
the keg. If the zone is a cache zone, this means that UMA will not
attempt any memory allocations when allocating an item from the backend.
This is intended for use after a panic by netdump, but likely has other
applications.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D15184