Create own free list for each of the first 32 possible allocation sizes.
In case of 4K allocation quantum that means for allocations up to 128K.

With growth of memory fragmentation these lists may grow to quite a large
sizes (tenths and hundreds of thousands items).  Having in one list items
of different sizes in worst case may require full linear list traversal,
that may be very expensive.  Having lists for items of single size means
that unless user specify some alignment or border requirements (that are
very rare cases) first item found on the list should satisfy the request.

While running SPEC NFS benchmark on top of ZFS on 24-core machine with
84GB RAM this change reduces CPU time spent in vmem_xalloc() from 8%
and lock congestion spinning around it from 20% to invisible levels.
And that all is by the cost of just 26 more pointers per vmem instance.

If at some point our kernel will start to actively use KVA allocations
with odd sizes above 128K, something may need to be done to bigger lists
also.