inet: constify inet_rtx_syn_ack() sock argument

SYNACK packets are sent on behalf on unlocked listeners
or fastopen sockets. Mark socket as const to catch future changes
that might break the assumption.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/request_sock.h

@@ -42,7 +42,7 @@ struct request_sock_ops {
 	void		(*syn_ack_timeout)(const struct request_sock *req);
 };
 
-int inet_rtx_syn_ack(struct sock *parent, struct request_sock *req);
+int inet_rtx_syn_ack(const struct sock *parent, struct request_sock *req);
 
 /* struct request_sock - mini sock to represent a connection request
  */

net/ipv4/inet_connection_sock.c

@@ -563,7 +563,7 @@ static inline void syn_ack_recalc(struct request_sock *req, const int thresh,
 		  req->num_timeout >= rskq_defer_accept - 1;
 }
 
-int inet_rtx_syn_ack(struct sock *parent, struct request_sock *req)
+int inet_rtx_syn_ack(const struct sock *parent, struct request_sock *req)
 {
 	int err = req->rsk_ops->rtx_syn_ack(parent, req);