selftests: net: fix array_size.cocci warning

Fit the following coccicheck warning:
tools/testing/selftests/net/reuseport_bpf_numa.c:89:28-29:
WARNING: Use ARRAY_SIZE.

It has been tested with gcc (Debian 8.3.0-6) 8.3.0 on x86_64.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>