bfdd: update vrf of received packet

bfdd: update vrf of received packet

on vrf-lite environment, all incoming bfd packets are received by the
same socket on the default namespace. the vrfid is not relevant and
needs to be updated based on the incoming interface where traffic has
been received. If the traffic is received from an interface belonging to
a separate vrf, update the vrfid value accordingly.

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