bpf: Add gpl_compatible flag to struct bpf_prog_info

Adding gpl_compatible flag to struct bpf_prog_info
so it can be dumped via bpf_prog_get_info_by_fd and
displayed via bpftool progs dump.

Alexei noticed 4-byte hole in struct bpf_prog_info,
so we put the u32 flags field in there, and we can
keep adding bit fields in there without breaking
user space.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>