IPVS: Fallback if persistence engine fails

Fall back to normal persistence handling if the persistence
engine fails to recognise a packet.

This way, at least the packet will go somewhere.

It is envisaged that iptables could be used to block packets
such if this is not desired although nf_conntrack_sip would
likely need to be enhanced first.

Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: Julian Anastasov <ja@ssi.bg>