Kegs with no items reserved have uk_reserve = 0. So the check
keg->uk_reserve >= dom->ud_free_items will be true once all slabs are
depleted. Then, rather than go and allocate a fresh slab, we return.
The intent was to do this only when the keg actually has a reserve, so
modify the check to verify this first. Another approach would be to
make uk_reserve signed and set it to -1 until uma_zone_reserve() is
called, but requires a few casts elsewhere.
The bug would cause us to go to the keg more often than necessary when
filling buckets. In a steady state I wouldn't expect it to make much of
a difference, but maybe I'm missing something. I noticed the problem by
code inspection.
Fixes: 1b2dcc8c54a8 ("uma: Avoid depleting keg reserves when filling a bucket")