tcp/dccp: constify rtx_synack() and friends

This is done to make sure we do not change listener socket
while sending SYNACK packets while socket lock is not held.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/request_sock.h b/include/net/request_sock.h
index 87935ca..ff7ce1e 100644
--- a/include/net/request_sock.h
+++ b/include/net/request_sock.h
@@ -32,7 +32,7 @@
 	int		obj_size;
 	struct kmem_cache	*slab;
 	char		*slab_name;
-	int		(*rtx_syn_ack)(struct sock *sk,
+	int		(*rtx_syn_ack)(const struct sock *sk,
 				       struct request_sock *req);
 	void		(*send_ack)(struct sock *sk, struct sk_buff *skb,
 				    struct request_sock *req);