libbpf: Fix array_size.cocci warning

Fix the following coccicheck warning:
tools/lib/bpf/bpf.c:114:31-32: WARNING: Use ARRAY_SIZE
tools/lib/bpf/xsk.c:484:34-35: WARNING: Use ARRAY_SIZE
tools/lib/bpf/xsk.c:485:35-36: 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: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220306023426.19324-1-guozhengkui@vivo.com