rcu/kvfree: Move need_offload_krc() out of krcp->lock

The need_offload_krc() function currently holds the krcp->lock in order
to safely check krcp->head.  This commit removes the need for this lock
in that function by updating the krcp->head pointer using WRITE_ONCE()
macro so that readers can carry out lockless loads of that pointer.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>