uma: Respect uk_reserve in keg_drain()

When a reserve of free items is configured for a zone, the reserve must
not be reclaimed under memory pressure. Modify keg_drain() to simply
respect the reserved pool.

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.

Reviewed by: alc, rlibby
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26772