uma: grow slabs to enforce minimum memory efficiency

Memory efficiency can be poor with awkward item sizes (e.g. 1/2 or 1
page size + epsilon). In order to achieve a minimum memory efficiency,
select a slab size with a potentially larger number of pages if it
yields a lower portion of waste.

This may mean using page_alloc instead of uma_small_alloc, which could
be more costly.

Discussed with: jeff, mckusick
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23239