rcu: Use WRITE_ONCE in RCU_INIT_POINTER

For the paranoid amongst us GCC would be in its right to use byte stores
to write our NULL value, tell it not to do that.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>