bpf: Refactor ARRAY_SIZE macro to bpf_util.h

This patch refactors the ARRAY_SIZE macro to bpf_util.h.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>