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: 6df96146

 ("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>