rcu: Allow processing deferred QSes for exiting RCU-preempt readers

If an RCU-preempt read-side critical section is exiting, that is,
->rcu_read_lock_nesting is negative, then it is a good time to look
at the possibility of reporting deferred quiescent states.  This
commit therefore updates the checks in rcu_preempt_need_deferred_qs()
to allow exiting critical sections to report deferred quiescent states.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -602,7 +602,7 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 {
 	return (this_cpu_ptr(&rcu_preempt_data)->deferred_qs ||
 		READ_ONCE(t->rcu_read_unlock_special.s)) &&
-	       !t->rcu_read_lock_nesting;
+	       t->rcu_read_lock_nesting <= 0;
 }
 
 /*