mm/slab: Move kvfree_rcu() into SLAB

Move kvfree_rcu() functionality to the slab_common.c file.

The reason to have kvfree_rcu() functionality as part of SLAB is that
there is a clear trend and need of closer integration. One of the recent
example is creating a barrier function for SLAB caches.

Another reason is to prevent of having several implementations of RCU
machinery for reclaiming objects after a GP. As future steps, it can be
more integrated(easier) with SLAB internals.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Acked-by: Hyeonggon Yoo <hyeonggon.yoo@sk.com>
Tested-by: Hyeonggon Yoo <hyeonggon.yoo@sk.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>