nat: fix forwarding handoff workaround

nat: fix forwarding handoff workaround

A special case when out2in packet needs to
be handoffed to other worker thread. We are
not able to determine which thread they belong
to in the first processing of nat handoff node.
These packets needs to go through out2in slowpath
before we are able to tell where to handoff them.

Type: fix
Ticket: VPP-1941

Change-Id: I7173bda970ce6a91d81f48fc72aa2457586a076f
Signed-off-by: Filip Varga <fivarga@cisco.com>