selftest: net: Assert on a proper value in so_incoming_cpu.c.

Dan Carpenter reported an error spotted by Smatch.

  ./tools/testing/selftests/net/so_incoming_cpu.c:163 create_clients()
  error: uninitialized symbol 'ret'.

The returned value of sched_setaffinity() should be checked with
ASSERT_EQ(), but the value was not saved in a proper variable,
resulting in an error above.

Let's save the returned value of with sched_setaffinity().

Fixes: 6df9614 ("selftest: Add test for SO_INCOMING_CPU.")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-kselftest/fe376760-33b6-4fc9-88e8-178e809af1ac@moroto.mountain/
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230731181553.5392-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/so_incoming_cpu.c

@@ -159,7 +159,7 @@ void create_clients(struct __test_metadata *_metadata,
 		/* Make sure SYN will be processed on the i-th CPU
 		 * and finally distributed to the i-th listener.
 		 */
-		sched_setaffinity(0, sizeof(cpu_set), &cpu_set);
+		ret = sched_setaffinity(0, sizeof(cpu_set), &cpu_set);
 		ASSERT_EQ(ret, 0);
 
 		for (j = 0; j < CLIENT_PER_SERVER; j++) {