uma dbg: flexible size for slab debug bitset too

Recently (r355315) the size of the struct uma_slab bitset field us_free
became dynamic instead of conservative. Now, make the debug bitset
size dynamic too. The debug bitset is INVARIANTS-only, so in fact we
don't care too much about the space savings that results from this, but
enabling minimally-sized slabs on INVARIANTS builds is still important
in order to be able to test new slab layouts effectively.

Reviewed by: jeff, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22759