When we set a reserve of free items for a zone, we must take care to avoid
reclamining them. Modify keg_drain() to simply respect the reserved pool. For
now we take uk_reserve to be a per-NUMA domain quantity, since that's what
existing code does.
While here remove an always-false uk_freef == NULL check (kegs that
shouldn't be drained should set _NOFREE instead), and make sure that the
keg_drain() KTR statement does not reference an uninitialized variable.