Now we will freeze slabs when moving them out of node partial list to
cpu partial list, this method needs two cmpxchg_double operations:

1. freeze slab (acquire_slab()) under the node list_lock
2. get_freelist() when pick used in ___slab_alloc()

Actually we don't need to freeze when moving slabs out of node partial
list, we can delay freezing to when use slab freelist in ___slab_alloc(),
so we can save one cmpxchg_double().

And there are other good points:
 - The moving of slabs between node partial list and cpu partial list
   becomes simpler, since we don't need to freeze or unfreeze at all.

 - The node list_lock contention would be less, since we don't need to
   freeze any slab under the node list_lock.

We can achieve this because there is no concurrent path would manipulate
the partial slab list except the __slab_free() path, which is now
serialized by slab_test_node_partial() under the list_lock.

Since the slab returned by get_partial() interfaces is not frozen anymore
and no freelist is returned in the partial_context, so we need to use the
introduced freeze_slab() to freeze it and get its freelist.

Similarly, the slabs on the CPU partial list are not frozen anymore,
we need to freeze_slab() on it before use.

We can now delete acquire_slab() as it became unused.
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>