ipvs: fix sync_threshold description and add sync_refresh_period, sync_retries

Fix sync_threshold description which should have two values. Also add
sync_refresh_period and sync_retries based on commit 749c42b620a9
("ipvs: reduce sync rate with time thresholds").

Signed-off-by: Hangbin Liu <[e-mail]>
Signed-off-by: Simon Horman <[e-mail]>

Documentation/networking/ipvs-sysctl.txt

···
         The value definition is the same as that of drop_entry and
         drop_packet.
 
-sync_threshold - INTEGER
-        default 3
+sync_threshold - vector of 2 INTEGERs: sync_threshold, sync_period
+	default 3 50
 
-        It sets synchronization threshold, which is the minimum number
-        of incoming packets that a connection needs to receive before
-        the connection will be synchronized. A connection will be
-        synchronized, every time the number of its incoming packets
-        modulus 50 equals the threshold. The range of the threshold is
-        from 0 to 49.
+	It sets synchronization threshold, which is the minimum number
+	of incoming packets that a connection needs to receive before
+	the connection will be synchronized. A connection will be
+	synchronized, every time the number of its incoming packets
+	modulus sync_period equals the threshold. The range of the
+	threshold is from 0 to sync_period.
+
+	When sync_period and sync_refresh_period are 0, send sync only
+	for state changes or only once when pkts matches sync_threshold
+
+sync_refresh_period - UNSIGNED INTEGER
+	default 0
+
+	In seconds, difference in reported connection timer that triggers
+	new sync message. It can be used to avoid sync messages for the
+	specified period (or half of the connection timeout if it is lower)
+	if connection state is not changed since last sync.
+
+	This is useful for normal connections with high traffic to reduce
+	sync rate. Additionally, retry sync_retries times with period of
+	sync_refresh_period/8.
+
+sync_retries - INTEGER
+	default 0
+
+	Defines sync retries with period of sync_refresh_period/8. Useful
+	to protect against loss of sync messages. The range of the
+	sync_retries is from 0 to 3.
 
 snat_reroute - BOOLEAN
 	0 - disabled