uma: split slabzone into two sizes

By allowing more items per slab, we can improve memory efficiency for
small allocs. If we were just to increase the bitmap size of the
slabzone, we would then waste slabzone memory. So, split slabzone into
two zones, one especially for 8-byte allocs (512 per slab). The
practical effect should be reduced memory usage for counter(9).

Reviewed by: jeff, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23149