rcu/kvfree: Initialize kvfree_rcu() separately

Introduce a separate initialization of kvfree_rcu() functionality.
For such purpose a kfree_rcu_batch_init() is renamed to a kvfree_rcu_init()
and it is invoked from the main.c right after rcu_init() is done.

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>