selftests/proc: fix array_size.cocci warning

Fix the following coccicheck warning:

tools/testing/selftests/proc/proc-pid-vm.c:371:26-27:
WARNING: Use ARRAY_SIZE
tools/testing/selftests/proc/proc-pid-vm.c:420:26-27:
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: Shuah Khan <skhan@linuxfoundation.org>