bpftool: Replace sizeof(arr)/sizeof(arr[0]) with ARRAY_SIZE macro

Use the ARRAY_SIZE macro and make the code more compact.

Signed-off-by: Rongguang Wei <weirongguang@kylinos.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220726093045.3374026-1-clementwei90@163.com