bpf: btf: Check array->index_type

Instead of ingoring the array->index_type field.  Enforce that
it must be a BTF_KIND_INT in size 1/2/4/8 bytes.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>