ipvs: fix crash with sync protocol v0 and FTP

[ Upstream commit 56184858d1fc95c46723436b455cb7261cd8be6f ]

Fix crash in 3.5+ if FTP is used after switching
sync_version to 0.

Fixes: 749c42b6 ("ipvs: reduce sync rate with time thresholds")
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>