mpls: fix forwarding using v4/v6 explicit null

Fill in the via address length for the predefined IPv4 and IPv6
explicit-null label routes.

Fixes: f8efb73c ("mpls: multipath route support")
Signed-off-by: Robert Shearman <rshearma@brocade.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>