rcu/kvfree: Adjust a shrinker name

Rename "rcu-kfree" to "slab-kvfree-rcu" since it goes to the
slab_common.c file soon.

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>