ipvs: avoid lookup for fwmark 0

 	Restore the previous behaviour to lookup for fwmark
service only when fwmark is non-null. This saves only CPU.

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>