bpf: btf: Validate type reference

After collecting all btf_type in the first pass in an earlier patch,
the second pass (in this patch) can validate the reference types
(e.g. the referring type does exist and it does not refer to itself).

While checking the reference type, it also gathers other information (e.g.
the size of an array).  This info will be useful in checking the
struct's members in a later patch.  They will also be useful in doing
pretty print later.

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