inet: add rsk_refcnt/ireq_refcnt to request socks

When request socks will be in ehash, they'll need to be refcounted.

This patch adds rsk_refcnt/ireq_refcnt macros, and adds
reqsk_put() function, but nothing yet use them.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/inet_sock.h

@@ -81,6 +81,7 @@ struct inet_request_sock {
 #define ir_cookie		req.__req_common.skc_cookie
 #define ireq_net		req.__req_common.skc_net
 #define ireq_state		req.__req_common.skc_state
+#define ireq_refcnt		req.__req_common.skc_refcnt
 
 	kmemcheck_bitfield_begin(flags);
 	u16			snd_wscale : 4,

include/net/request_sock.h

@@ -49,6 +49,8 @@ int inet_rtx_syn_ack(struct sock *parent, struct request_sock *req);
  */
 struct request_sock {
 	struct sock_common		__req_common;
+#define rsk_refcnt			__req_common.skc_refcnt
+
 	struct request_sock		*dl_next;
 	u16				mss;
 	u8				num_retrans; /* number of retransmits */
@@ -86,6 +88,12 @@ static inline void reqsk_free(struct request_sock *req)
 	__reqsk_free(req);
 }
 
+static inline void reqsk_put(struct request_sock *req)
+{
+	if (atomic_dec_and_test(&req->rsk_refcnt))
+		reqsk_free(req);
+}
+
 extern int sysctl_max_syn_backlog;
 
 /** struct listen_sock - listen state