net: mpls: Add support for netconf

Add netconf support to MPLS. Allows userpsace to learn and be notified
of changes to 'input' enable setting per interface.

Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>