rcu/kvfree: Add kvfree_rcu_barrier() API

Add a kvfree_rcu_barrier() function. It waits until all
in-flight pointers are freed over RCU machinery. It does
not wait any GP completion and it is within its right to
return immediately if there are no outstanding pointers.

This function is useful when there is a need to guarantee
that a memory is fully freed before destroying memory caches.
For example, during unloading a kernel module.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>