bpf: btf: Check members of struct/union

This patch checks a few things of struct's members:

1) It has a valid size (e.g. a "const void" is invalid)
2) A member's size (+ its member's offset) does not exceed
   the containing struct's size.
3) The member's offset satisfies the alignment requirement

The above can only be done after the needs_resolve member's type
is resolved.  Hence, the above is done together in
btf_struct_resolve().

Each possible member's type (e.g. int, enum, modifier...) implements
the check_member() ops which will be called from btf_struct_resolve().

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