dccp: constify dccp_make_response() socket argument

Like tcp_make_synack() the only time we might change the socket is
when calling sock_wmalloc(), which is using atomic operation to
update sk->sk_wmem_alloc

Also use MAX_DCCP_HEADER as both IPv4/IPv6 use this value for max_header.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/dccp.h

@@ -293,7 +293,7 @@ int dccp_init_sock(struct sock *sk, const __u8 ctl_sock_initialized);
 void dccp_destroy_sock(struct sock *sk);
 
 void dccp_close(struct sock *sk, long timeout);
-struct sk_buff *dccp_make_response(struct sock *sk, struct dst_entry *dst,
+struct sk_buff *dccp_make_response(const struct sock *sk, struct dst_entry *dst,
 				   struct request_sock *req);
 
 int dccp_connect(struct sock *sk);

net/dccp/output.c

@@ -390,7 +390,7 @@ int dccp_retransmit_skb(struct sock *sk)
 	return dccp_transmit_skb(sk, skb_clone(sk->sk_send_head, GFP_ATOMIC));
 }
 
-struct sk_buff *dccp_make_response(struct sock *sk, struct dst_entry *dst,
+struct sk_buff *dccp_make_response(const struct sock *sk, struct dst_entry *dst,
 				   struct request_sock *req)
 {
 	struct dccp_hdr *dh;
@@ -398,13 +398,18 @@ struct sk_buff *dccp_make_response(struct sock *sk, struct dst_entry *dst,
 	const u32 dccp_header_size = sizeof(struct dccp_hdr) +
 				     sizeof(struct dccp_hdr_ext) +
 				     sizeof(struct dccp_hdr_response);
-	struct sk_buff *skb = sock_wmalloc(sk, sk->sk_prot->max_header, 1,
-					   GFP_ATOMIC);
-	if (skb == NULL)
+	struct sk_buff *skb;
+
+	/* sk is marked const to clearly express we dont hold socket lock.
+	 * sock_wmalloc() will atomically change sk->sk_wmem_alloc,
+	 * it is safe to promote sk to non const.
+	 */
+	skb = sock_wmalloc((struct sock *)sk, MAX_DCCP_HEADER, 1,
+			   GFP_ATOMIC);
+	if (!skb)
 		return NULL;
 
-	/* Reserve space for headers. */
-	skb_reserve(skb, sk->sk_prot->max_header);
+	skb_reserve(skb, MAX_DCCP_HEADER);
 
 	skb_dst_set(skb, dst_clone(dst));