nat: fix dslite session port allocation
nat: fix dslite session port allocation

Fix allocation of port per dslite session.
After each session is created per protocol,
when new one should be created with new port,
instead it's trying to create with the same port
and while(1) loop is executed forever and VPP does
not response

Type: fix

Signed-off-by: Vladimir Ratnikov <0b4540b213bf6184fb7926787f1863fd00af24ce@netgate.com>
Change-Id: Ic91b8b07253498ef9846ca60bcd4c4c76a5fac91

FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp