rcu: Fix exp_funnel_lock()/rcu_exp_wait_wake() datarace

The rcu_node structure's ->exp_seq_rq field is accessed locklessly, so
updates must use WRITE_ONCE().  This commit therefore adds the needed
WRITE_ONCE() invocation where it was missed.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_exp.h

@@ -589,7 +589,7 @@ static void rcu_exp_wait_wake(unsigned long s)
 			spin_lock(&rnp->exp_lock);
 			/* Recheck, avoid hang in case someone just arrived. */
 			if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
-				rnp->exp_seq_rq = s;
+				WRITE_ONCE(rnp->exp_seq_rq, s);
 			spin_unlock(&rnp->exp_lock);
 		}
 		smp_mb(); /* All above changes before wakeup. */