libbpf: fix btf_dedup equivalence check handling of different kinds

btf_dedup_is_equiv() used to compare btf_type->info fields, before doing
kind-specific equivalence check. This comparsion implicitly verified
that candidate and canonical types are of the same kind. With enum fwd
resolution logic this check couldn't be done generically anymore, as for
enums info contains vlen, which differs between enum fwd and
fully-defined enum, so this check was subsumed by kind-specific
equivalence checks.

This change caused btf_dedup_is_equiv() to let through VOID vs other
types check to reach switch, which was never meant to be handing VOID
kind, as VOID kind is always pre-resolved to itself and is only
equivalent to itself, which is checked early in btf_dedup_is_equiv().

This change adds back BTF kind equality check in place of more generic
btf_type->info check, still defering further kind-specific checks to
a per-kind switch.

Fixes: 9768095b ("btf: resolve enum fwds in btf_dedup")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>