cnat: Fix conflicting rsession

cnat: Fix conflicting rsession

When dNAT-ing to a VIP, it can happen
that the return session conflicts with
another forward session than the one
we own.

This patchs adds a rsession_flags
CNAT_SESSION_RETRY_SNAT that makes cnat_session_create
search for a free src port to use for the
resulting return session.

It also makes forward & return session
share their fate in the session scanner.

Type: fix

Change-Id: Id0edf59abf8e5bc0c0d8941ba289c4563c77dee0
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>