libbpf: BTF dumper support for typed data

Add a BTF dumper for typed data, so that the user can dump a typed
version of the data provided.

The API is

int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
                             void *data, size_t data_sz,
                             const struct btf_dump_type_data_opts *opts);

...where the id is the BTF id of the data pointed to by the "void *"
argument; for example the BTF id of "struct sk_buff" for a
"struct skb *" data pointer.  Options supported are

 - a starting indent level (indent_lvl)
 - a user-specified indent string which will be printed once per
   indent level; if NULL, tab is chosen but any string <= 32 chars
   can be provided.
 - a set of boolean options to control dump display, similar to those
   used for BPF helper bpf_snprintf_btf().  Options are
        - compact : omit newlines and other indentation
        - skip_names: omit member names
        - emit_zeroes: show zero-value members

Default output format is identical to that dumped by bpf_snprintf_btf(),
for example a "struct sk_buff" representation would look like this:

struct sk_buff){
	(union){
		(struct){
			.next = (struct sk_buff *)0xffffffffffffffff,
			.prev = (struct sk_buff *)0xffffffffffffffff,
		(union){
			.dev = (struct net_device *)0xffffffffffffffff,
			.dev_scratch = (long unsigned int)18446744073709551615,
		},
	},
...

If the data structure is larger than the *data_sz*
number of bytes that are available in *data*, as much
of the data as possible will be dumped and -E2BIG will
be returned.  This is useful as tracers will sometimes
not be able to capture all of the data associated with
a type; for example a "struct task_struct" is ~16k.
Being able to specify that only a subset is available is
important for such cases.  On success, the amount of data
dumped is returned.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1626362126-27775-2-git-send-email-alan.maguire@oracle.com