Merge from projects/counters: UMA_ZONE_PCPU zones.

  These zones have slab size == sizeof(struct pcpu), but request from VM
enough pages to fit (uk_slabsize * mp_ncpus). An item allocated from such
zone would have a separate twin for each CPU in the system, and these twins
are at a distance of sizeof(struct pcpu) from each other. This magic value
of distance would allow us to make some optimizations later.

  To address private item from a CPU simple arithmetics should be used:

  item = (type *)((char *)base + sizeof(struct pcpu) * curcpu)

  These arithmetics are available as zpcpu_get() macro in pcpu.h.

  To introduce non-page size slabs a new field had been added to uma_keg
uk_slabsize. This shifted some frequently used fields of uma_keg to the
fourth cache line on amd64. To mitigate this pessimization, uma_keg fields
were a bit rearranged and least frequently used uk_name and uk_link moved
down to the fourth cache line. All other fields, that are dereferenced
frequently fit into first three cache lines.

Sponsored by:	Nginx, Inc.

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