bgpd: update the mpls entry to handle return traffic

bgpd: update the mpls entry to handle return traffic

When advertising an mpls vpn entry with a new label,
the return traffic is redirected to the local machine,
but the MPLS traffic is dropped.

Add an MPLS entry to handle MPLS packets which have
the new label value. Traffic is swapped to the original
label value from the mpls vpn next-hop entry; then it is
sent to the resolved next-hop of the original next-hop
from the mpls vpn next-hop entry.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>