IPVS: Add persistence engine data to /proc/net/ip_vs_conn

This shouldn't break compatibility with userspace as the new data
is at the end of the line.

I have confirmed that this doesn't break ipvsadm, the main (only?)
user-space user of this data.

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