zebra: inform upper layer error when reading correct speed interface

zebra: inform upper layer error when reading correct speed interface

speed interface is done 15 seconds after interface creation. during that
time, the vrf or the interface may have disappeared. to protect this,
return an error in case it is not possible to create a vrf socket or it
is not possible to get speed of an interface because of a missing
device.

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