Reduce lock hold time in keg_drain().

Maintain a count of free slabs in the per-domain keg structure and use
that to clear the free slab list in constant time for most cases. This
helps minimize lock contention induced by reclamation, in preparation
for proactive trimming of excesses of free memory.

Reviewed by: jeff, rlibby
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D23532