rcutorture: Warn on bad torture type for built-in tests

When running a built-in rcutorture test, specifying an invalid torture
type results in what looks like a hard hang, with the error messages
hidden by other boot-time output.  This commit therefore executes a
WARN_ON() in this case so that the splat appears just following the
error messages.

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

kernel/rcu/rcutorture.c

@@ -1968,6 +1968,7 @@ rcu_torture_init(void)
 		for (i = 0; i < ARRAY_SIZE(torture_ops); i++)
 			pr_cont(" %s", torture_ops[i]->name);
 		pr_cont("\n");
+		WARN_ON(!IS_MODULE(CONFIG_RCU_TORTURE_TEST));
 		firsterr = -EINVAL;
 		goto unwind;
 	}