zebra: fix mpls config on ifaces created post frr

zebra: fix mpls config on ifaces created post frr

The mpls configuration does not work when an interface is
created after having applied the frr configuration. The
below scenario illustrates:

> root@dut:~# modprobe mpls
> root@dut:~# zebra &
> [..]
> dut(config)# interface ifacenotcreated
> dut(config-if)# mpls enable
> dut(config-if)# Ctrl-D
> root@dut:~# ip li show ifacenotcreated
> Device "ifacenotcreated" does not exist.
> root@dut:~# ip li add ifacenotcreated type dummy
> 0

Fix this by forcing the mpls flag when the interface is detected.

> root@dut:~# cat /proc/sys/net/mpls/conf/ifacenotcreat/input
> 1

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