rcutorture: Suppressing read-exit testing is not an error

Currently, specifying the rcutorture.read_exit_burst=0 kernel boot
parameter will result in a -EINVAL exit code that will stop the rcutorture
test run before it has fully initialized.  This commit therefore uses a
zero exit code in that case, thus allowing rcutorture.read_exit_burst=0
to complete normally.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcutorture.c

@@ -2741,7 +2741,7 @@ static int rcu_torture_read_exit(void *unused)
 static int rcu_torture_read_exit_init(void)
 {
 	if (read_exit_burst <= 0)
-		return -EINVAL;
+		return 0;
 	init_waitqueue_head(&read_exit_wq);
 	read_exit_child_stop = false;
 	read_exit_child_stopped = false;