ipvs: fix sync_threshold description and add sync_refresh_period, sync_retries

        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.

	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