bpf: Define struct bpf_tcp_req_attrs when CONFIG_SYN_COOKIES=n.

kernel test robot reported the warning below:

  >> net/core/filter.c:11842:13: warning: declaration of 'struct bpf_tcp_req_attrs' will not be visible outside of this function [-Wvisibility]
      11842 |                                         struct bpf_tcp_req_attrs *attrs, int attrs__sz)
            |                                                ^
     1 warning generated.

struct bpf_tcp_req_attrs is defined under CONFIG_SYN_COOKIES
but used in kfunc without the config.

Let's move struct bpf_tcp_req_attrs definition outside of
CONFIG_SYN_COOKIES guard.

Fixes: e472f888 ("bpf: tcp: Support arbitrary SYN Cookie.")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401180418.CUVc0hxF-lkp@intel.com/Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240118211751.25790-1-kuniyu@amazon.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/net/tcp.h

@@ -498,6 +498,22 @@ struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
 					    struct tcp_options_received *tcp_opt,
 					    int mss, u32 tsoff);
 
+#if IS_ENABLED(CONFIG_BPF)
+struct bpf_tcp_req_attrs {
+	u32 rcv_tsval;
+	u32 rcv_tsecr;
+	u16 mss;
+	u8 rcv_wscale;
+	u8 snd_wscale;
+	u8 ecn_ok;
+	u8 wscale_ok;
+	u8 sack_ok;
+	u8 tstamp_ok;
+	u8 usec_ts_ok;
+	u8 reserved[3];
+};
+#endif
+
 #ifdef CONFIG_SYN_COOKIES
 
 /* Syncookies use a monotonic timer which increments every 60 seconds.
@@ -600,20 +616,6 @@ static inline bool cookie_ecn_ok(const struct net *net, const struct dst_entry *
 }
 
 #if IS_ENABLED(CONFIG_BPF)
-struct bpf_tcp_req_attrs {
-	u32 rcv_tsval;
-	u32 rcv_tsecr;
-	u16 mss;
-	u8 rcv_wscale;
-	u8 snd_wscale;
-	u8 ecn_ok;
-	u8 wscale_ok;
-	u8 sack_ok;
-	u8 tstamp_ok;
-	u8 usec_ts_ok;
-	u8 reserved[3];
-};
-
 static inline bool cookie_bpf_ok(struct sk_buff *skb)
 {
 	return skb->sk;