softlockup: convert read_lock in hung_task to rcu_read_lock

Since the tasklist is protected by rcu list operations, it is safe
to convert the read_lock()s to rcu_read_lock().

Suggested-by: Peter Zijlstra <[e-mail]>
Signed-off-by: Mandeep Singh Baines <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/hung_task.c

···
 	if (test_taint(TAINT_DIE) || did_panic)
 		return;
 
-	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	do_each_thread(g, t) {
 		if (!--max_count)
 			goto unlock;
···
 			check_hung_task(t, now, timeout);
 	} while_each_thread(g, t);
  unlock:
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 }
 
 static void update_poll_jiffies(void)