rcu: Make expedited IPI handler return after handling critical section

During expedited RCU grace-period initialization, IPIs are sent to
all non-idle online CPUs.  The IPI handler checks to see if the CPU is
in quiescent state, reporting one if so.  This handler looks at three
different cases: (1) The CPU is not in an rcu_read_lock()-based critical
section, (2) The CPU is in the process of exiting an rcu_read_lock()-based
critical section, and (3) The CPU is in an rcu_read_lock()-based critical
section.  In case (2), execution falls through into case (3).

This is harmless from a functionality viewpoint, but can result in
needless overhead during an improbable corner case.  This commit therefore
adds the "return" statement needed to prevent fall-through.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree_exp.h

@@ -697,6 +697,7 @@ static void sync_rcu_exp_handler(void *unused)
 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
 		}
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+		return;
 	}
 
 	/*