bpf: fix "unresolved symbol" build error with resolve_btfids

Michal reported a build failure likes below:
   BTFIDS  vmlinux
   FAILED unresolved symbol tcp_timewait_sock
   make[1]: *** [/.../linux-5.9-rc7/Makefile:1176: vmlinux] Error 255

This error can be triggered when config has CONFIG_NET enabled
but CONFIG_INET disabled. In this case, there is no user of
structs inet_timewait_sock and tcp_timewait_sock and hence vmlinux BTF
types are not generated for these two structures.

To fix the problem, let us force BTF generation for these two
structures with BTF_TYPE_EMIT.

Fixes: fce557b ("bpf: Make btf_sock_ids global")
Reported-by: Michal Kubecek <[email protected]>
Signed-off-by: Yonghong Song <[email protected]>

Author: yonghong-song

net/core/filter.c

@@ -9558,6 +9558,12 @@ const struct bpf_func_proto bpf_skc_to_tcp_sock_proto = {
 
 BPF_CALL_1(bpf_skc_to_tcp_timewait_sock, struct sock *, sk)
 {
+	/* BTF types for tcp_timewait_sock and inet_timewait_sock are not
+	 * generated if CONFIG_INET=n. Trigger an explicit generation here.
+	 */
+	BTF_TYPE_EMIT(struct inet_timewait_sock);
+	BTF_TYPE_EMIT(struct tcp_timewait_sock);
+
 #ifdef CONFIG_INET
 	if (sk && sk->sk_prot == &tcp_prot && sk->sk_state == TCP_TIME_WAIT)
 		return (unsigned long)sk;