bpf: decouple btf from seq bpf fs dump and enable more maps

Commit a26ca7c982 ("bpf: btf: Add pretty print support to
the basic arraymap") and 699c86d6ec ("bpf: btf: add pretty
print for hash/lru_hash maps") enabled support for BTF and
dumping via BPF fs for array and hash/lru map. However, both
can be decoupled from each other such that regular BPF maps
can be supported for attaching BTF key/value information,
while not all maps necessarily need to dump via map_seq_show_elem()
callback.

The basic sanity check which is a prerequisite for all maps
is that key/value size has to match in any case, and some maps
can have extra checks via map_check_btf() callback, e.g.
probing certain types or indicating no support in general. With
that we can also enable retrieving BTF info for per-cpu map
types and lpm.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>

kernel/bpf/stackmap.c

@@ -607,6 +607,7 @@ const struct bpf_map_ops stack_map_ops = {
 	.map_lookup_elem = stack_map_lookup_elem,
 	.map_update_elem = stack_map_update_elem,
 	.map_delete_elem = stack_map_delete_elem,
+	.map_check_btf = map_check_no_btf,
 };
 
 static int __init stack_map_init(void)