rcu: Remove dead code from force_qs_rnp()

Because force_qs_rnp() is invoked only from the force-quiescent-state
code which runs only in the context of the grace-period kthread, a grace
period must always be in progress throughout force_qs_rnp()'s execution.
This commit therefore removes the rcu_gp_in_progress() check and the
associated dead code.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -2720,10 +2720,6 @@ static void force_qs_rnp(struct rcu_state *rsp,
 		mask = 0;
 		raw_spin_lock_irqsave(&rnp->lock, flags);
 		smp_mb__after_unlock_lock();
-		if (!rcu_gp_in_progress(rsp)) {
-			raw_spin_unlock_irqrestore(&rnp->lock, flags);
-			return;
-		}
 		if (rnp->qsmask == 0) {
 			if (rcu_state_p == &rcu_sched_state ||
 			    rsp != rcu_state_p ||