Right now, UMA uses a hand-rolled round-robin policy. Aside from the
duplication of logic, this means that we're not following the "minskip"
policy introduced in r338507. We also want to avoid blocking in
individual domains with a WAITOK allocation. Thus, refactor
keg_fetch_slab() to make use of a vm_domainset iterator instead.
I changed keg_alloc_slab() to return with the keg unlocked on failure,
identical to zone_fetch_slab_multi(). I also made changes to ensure
that uk_reserve is only accessed with the keg lock held.
One side effect of this is that UMA allocations are now round-robin by
thread rather than round-robin by keg. I don't believe this will matter