mm, slub: detach whole partial list at once in unfreeze_partials()

mm, slub: detach whole partial list at once in unfreeze_partials()

Instead of iterating through the live percpu partial list, detach it from the
kmem_cache_cpu at once. This is simpler and will allow further optimization.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>