addons: vrf: fix link-down yes on vrf throws an exception

addons: vrf: fix link-down yes on vrf throws an exception

before the patch we can see that ifreload is displaying an error
with the example config

$ ifquery -a
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp

auto swp1
iface swp1
      vrf blue
      	  link-down yes

auto blue
iface blue
      vrf-table auto

$ ifreload -a
error: eth0: vrf blue not around, skipping vrf config
$ echo $?
1
$

$ # applying patch
$ ifreload -a
$ echo $?
0

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>