selftests/bpf: Relax TCPOPT_WINDOW validation in test_tcp_custom_syncookie.c.

The custom syncookie test expects TCPOPT_WINDOW to be 7 based on the
kernel’s behaviour at the time, but the upcoming series [0] will bump
it to 10.

Let's relax the test to allow any valid TCPOPT_WINDOW value in the
range 1–14.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/netdev/20250513193919.1089692-1-edumazet@google.com/ #[0]
Link: https://patch.msgid.link/20250514214021.85187-1-kuniyu@amazon.com

Author: q2ven

tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c

@@ -294,7 +294,9 @@ static int tcp_validate_sysctl(struct tcp_syncookie *ctx)
 	    (ctx->ipv6 && ctx->attrs.mss != MSS_LOCAL_IPV6))
 		goto err;
 
-	if (!ctx->attrs.wscale_ok || ctx->attrs.snd_wscale != 7)
+	if (!ctx->attrs.wscale_ok ||
+	    !ctx->attrs.snd_wscale ||
+	    ctx->attrs.snd_wscale >= BPF_SYNCOOKIE_WSCALE_MASK)
 		goto err;
 
 	if (!ctx->attrs.tstamp_ok)