bpf: btf: Add pretty print capability for data with BTF type info

This patch adds pretty print capability for data with BTF type info.
The current usage is to allow pretty print for a BPF map.

The next few patches will allow a read() on a pinned map with BTF
type info for its key and value.

This patch uses the seq_printf() infra.

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