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>
diff --git a/include/net/request_sock.h b/include/net/request_sock.h
index ff7ce1e..181f97f 100644
--- a/include/net/request_sock.h
+++ b/include/net/request_sock.h
@@ -42,7 +42,7 @@
 	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
  */