kselftest/arm64: fix array_size.cocci warning

Fix the following coccicheck warnings:
tools/testing/selftests/arm64/mte/check_child_memory.c:110:25-26:
WARNING: Use ARRAY_SIZE
tools/testing/selftests/arm64/mte/check_child_memory.c:88:24-25:
WARNING: Use ARRAY_SIZE
tools/testing/selftests/arm64/mte/check_child_memory.c:90:20-21:
WARNING: Use ARRAY_SIZE
tools/testing/selftests/arm64/mte/check_child_memory.c:147:24-25:
WARNING: Use ARRAY_SIZE

`ARRAY_SIZE` macro is defined in tools/testing/selftests/kselftest.h.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220419032501.22790-1-guozhengkui@vivo.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>