selftests: net: fix array_size.cocci warning

Fix array_size.cocci warning in tools/testing/selftests/net.

Use `ARRAY_SIZE(arr)` instead of forms like `sizeof(arr)/sizeof(arr[0])`.

It has been tested with gcc (Debian 8.3.0-6) 8.3.0.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Link: https://lore.kernel.org/r/20220316092858.9398-1-guozhengkui@vivo.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>