ipvs: prefer NETDEV_DOWN event to free cached dsts

The real server becomes unreachable on down event,
no need to wait device unregistration. Should help in
releasing dsts early before dst->dev is replaced with lo.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off by: Hans Schillstrom <hans@schillstrom.com>
Signed-off-by: Simon Horman <horms@verge.net.au>