When destroying a UMA zone which has a reserve (set with
uma_zone_reserve()), messages like the following appear on the console:
"Freed UMA keg (Test zone) was not empty (0 items). Lost 528 pages of
memory."

When keg_drain_domain() is draining the zone, it tries to keep the number
of items specified in the reservation. However, when we are destroying the
UMA zone, we do not need to keep those items. Therefore, when destroying a
non-secondary and non-cache zone, we should reset the keg reservation to 0
prior to draining the zone.

Reviewed by: markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D27129