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 749c42b6
("ipvs: reduce sync rate with time thresholds").

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>