Introduce the UMA_ZONE_NOTRIM uma zone type

The ktls buffer zone allocates 16k contiguous buffers, and often needs
to call vm_page_reclaim_contig_domain_ext() to free up contiguous
memory, which can be expensive. Web servers which have a daily
pattern of peaks and troughs end up having UMA trim the
ktls_buffer_zone when they are in their trough, and end up re-building
it on the way to their peak.

Rather than calling vm_page_reclaim_contig_domain_ext() multiple times
on a daily basis, lets mark the ktls_buffer_zone with a new UMA flag,
UMA_ZONE_NOTRIM. This disables UMA_RECLAIM_TRIM on the zone, but
allows UMA_RECLAIM_DRAIN* operations, so that if we become extremely
short of memory (vm_page_count_severe()), the uma reclaim worker can
still free up memory.

Note that UMA_ZONE_UNMANAGED already exists, but can never be drained
or trimmed, so it may hold on to memory during times of severe memory
pressure. Using UMA_ZONE_NOTRIM rather than UMA_ZONE_UNMANAGED is an
attempt to keep this zone more reactive in the face of severe memory
pressure.

Sponsored by: Netflix
Reviewed by: jhb, kib, markj (via slack)
Differential Revision: https://reviews.freebsd.org/D48451