libbpf: Add support for prog_tracing

Cleanup libbpf from expected_attach_type == attach_btf_id hack
and introduce BPF_PROG_TYPE_TRACING.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191030223212.953010-3-ast@kernel.org

src/bpf.h

@@ -78,7 +78,10 @@ struct bpf_load_program_attr {
 	size_t insns_cnt;
 	const char *license;
 	__u32 kern_version;
-	__u32 prog_ifindex;
+	union {
+		__u32 prog_ifindex;
+		__u32 attach_btf_id;
+	};
 	__u32 prog_btf_fd;
 	__u32 func_info_rec_size;
 	const void *func_info;