tcpv6: fix error with CONFIG_TCP_MD5SIG disabled

This patch fix error with CONFIG_TCP_MD5SIG disabled.

Signed-off-by: Guo-Fu Tseng <cooldavid@cooldavid.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 13c6514..e5310c9b 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1036,9 +1036,7 @@
 {
 	struct tcphdr *th = tcp_hdr(skb);
 	u32 seq = 0, ack_seq = 0;
-#ifdef CONFIG_TCP_MD5SIG
-	struct tcp_md5sig_key *key;
-#endif
+	struct tcp_md5sig_key *key = NULL;
 
 	if (th->rst)
 		return;
@@ -1049,8 +1047,6 @@
 #ifdef CONFIG_TCP_MD5SIG
 	if (sk)
 		key = tcp_v6_md5_do_lookup(sk, &ipv6_hdr(skb)->daddr);
-	else
-		key = NULL;
 #endif
 
 	if (th->ack)