rcu/nocb: Never downgrade ->nocb_defer_wakeup in wake_nocb_gp_defer()

Currently, wake_nocb_gp_defer() simply stores whatever waketype was
passed in, which can result in a RCU_NOCB_WAKE_FORCE being downgraded
to RCU_NOCB_WAKE, which could in turn delay callback processing.
This commit therefore adds a check so that wake_nocb_gp_defer() only
updates ->nocb_defer_wakeup when the update increases the forcefulness,
thus avoiding downgrades.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree_plugin.h

@@ -1600,7 +1600,8 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
 {
 	if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT)
 		mod_timer(&rdp->nocb_timer, jiffies + 1);
-	WRITE_ONCE(rdp->nocb_defer_wakeup, waketype);
+	if (rdp->nocb_defer_wakeup < waketype)
+		WRITE_ONCE(rdp->nocb_defer_wakeup, waketype);
 	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, reason);
 }