rcuperf: Warn on bad perf type for built-in tests

When running a built-in rcuperf test, specifying an invalid perf 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/rcuperf.c

@@ -680,6 +680,7 @@ rcu_perf_init(void)
 		for (i = 0; i < ARRAY_SIZE(perf_ops); i++)
 			pr_cont(" %s", perf_ops[i]->name);
 		pr_cont("\n");
+		WARN_ON(!IS_MODULE(CONFIG_RCU_PERF_TEST));
 		firsterr = -EINVAL;
 		goto unwind;
 	}