ipvs: optimize release of connections in OPS mode

One-packet-scheduling is the most expensive mode in IPVS from
performance point of view: for each packet to be processed a new
connection data structure is created and, after packet is sent,
deleted by starting a new timer set to expire immediately.

SIP persistent-engine needs OPS mode to have Call-ID based load
balancing, so OPS mode performance has negative impact in SIP
protocol load balancing.

This patch aims to improve performance of OPS mode by means of the
following changes in the release mechanism of OPS connections:
a) call expire callback ip_vs_conn_expire() directly instead of
   starting a timer programmed to fire immediately.
b) avoid call_rcu() overhead inside expire callback, since OPS
   connection are not inserted in the hash-table and last just the
   time to process the packet, hence there is no concurrent access
   to such data structures.

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