rcutorture: Add missing return and use __func__ in warning

The rcutorture module has an rcu_torture_writer task that repeatedly
performs writes, synchronizations, and deletes. There is a corner-case
check in rcu_torture_writer() wherein if nsynctypes is 0, a warning is
issued and the task waits to be stopped via a call to
torture_kthread_stopping() rather than performing any work.

There should be a return statement following this call to
torture_kthread_stopping(), as the intention with issuing the call to
torture_kthread_stopping() in the first place is to avoid the
rcu_torture_writer task from performing any work. Some of the work may even
be dangerous to perform, such as potentially causing a #DE due to
nsynctypes being used in a modulo operator when querying for sync updates
to issue.

This patch adds the missing return call.  As a bonus, it also fixes a
checkpatch warning that was emitted due to the WARN_ONCE() call using the
name of the function rather than __func__.

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcutorture.c

@@ -1178,14 +1178,15 @@ rcu_torture_writer(void *arg)
 			 " GP expediting controlled from boot/sysfs for %s.\n",
 			 torture_type, cur_ops->name);
 	if (WARN_ONCE(nsynctypes == 0,
-		      "rcu_torture_writer: No update-side primitives.\n")) {
+		      "%s: No update-side primitives.\n", __func__)) {
 		/*
 		 * No updates primitives, so don't try updating.
 		 * The resulting test won't be testing much, hence the
 		 * above WARN_ONCE().
 		 */
 		rcu_torture_writer_state = RTWS_STOPPING;
 		torture_kthread_stopping("rcu_torture_writer");
+		return 0;
 	}
 
 	do {