rcu/kvfree: Adjust names passed into trace functions

Currently trace functions are supplied with "rcu_state.name"
member which is located in the structure. The problem is that
the "rcu_state" structure variable is local and can not be
accessed from another place.

To address this, this preparation patch passes "slab" string
as a first argument.

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>