kselftest/arm64: fix array_size.cocci warning

Use ARRAY_SIZE to fix the following coccicheck warnings:
tools/testing/selftests/arm64/mte/check_buffer_fill.c:341:20-21:
WARNING: Use ARRAY_SIZE
tools/testing/selftests/arm64/mte/check_buffer_fill.c:35:20-21:
WARNING: Use ARRAY_SIZE
tools/testing/selftests/arm64/mte/check_buffer_fill.c:168:20-21:
WARNING: Use ARRAY_SIZE
tools/testing/selftests/arm64/mte/check_buffer_fill.c:72:20-21:
WARNING: Use ARRAY_SIZE
tools/testing/selftests/arm64/mte/check_buffer_fill.c:369:25-26:
WARNING: Use ARRAY_SIZE

Signed-off-by: Kang Minchul <tegongkang@gmail.com>
Link: https://lore.kernel.org/r/20221105073143.78521-1-tegongkang@gmail.com


Signed-off-by: Will Deacon <will@kernel.org>