This is part of a series of patches intended to enable first-touch numa policies for UMA by default. It also reduces the cost of uma_zalloc/zfree by approximately 30% each in my tests.
Zone lock protected a few random things that did not provide any real protection so I eliminated them. Now zone lock effectively synchronizes the bucket lists and counts. So it is used in the cache_alloc()/cache_free() path but not below. These holds are all very short. If this lock is still shown to be contending we could further break the zone lock up by domain with a little effort in the bucket accounting.
This helps with crossdomain free because you may have to drain buckets back to the keg layer. This process will now not hold-up the normal allocation/free path. The introduction of uz_import/uz_release also broke any strong relationship between these two layers.