libbpf: Skip forward declaration when counting duplicated type names

Currently if a declaration appears in the BTF before the definition, the
definition is dumped as a conflicting name, e.g.:

    $ bpftool btf dump file vmlinux format raw | grep "'unix_sock'"
    [81287] FWD 'unix_sock' fwd_kind=struct
    [89336] STRUCT 'unix_sock' size=1024 vlen=14

    $ bpftool btf dump file vmlinux format c | grep "struct unix_sock"
    struct unix_sock;
    struct unix_sock___2 {	<--- conflict, the "___2" is unexpected
		    struct unix_sock___2 *unix_sk;

This causes a compilation error if the dump output is used as a header file.

Fix it by skipping declaration when counting duplicated type names.

Fixes: 351131b5 ("libbpf: add btf_dump API for BTF-to-C conversion")
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20220301053250.1464204-2-xukuohai@huawei.com

tools/lib/bpf/btf_dump.c

@@ -1505,6 +1505,11 @@ static const char *btf_dump_resolve_name(struct btf_dump *d, __u32 id,
 	if (s->name_resolved)
 		return *cached_name ? *cached_name : orig_name;
 
+	if (btf_is_fwd(t) || (btf_is_enum(t) && btf_vlen(t) == 0)) {
+		s->name_resolved = 1;
+		return orig_name;
+	}
+
 	dup_cnt = btf_dump_name_dups(d, name_map, orig_name);
 	if (dup_cnt > 1) {
 		const size_t max_len = 256;