inet: makes syn_ack_timeout mandatory

There are two struct request_sock_ops providers, tcp and dccp.

inet_csk_reqsk_queue_prune() can avoid testing syn_ack_timeout being
NULL if we make it non NULL like syn_ack_timeout

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Cc: dccp@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/dccp.h

@@ -549,6 +549,8 @@ static inline const char *dccp_role(const struct sock *sk)
 	return NULL;
 }
 
+extern void dccp_syn_ack_timeout(struct sock *sk, struct request_sock *req);
+
 #endif /* __KERNEL__ */
 
 #endif /* _LINUX_DCCP_H */