bgpd: update route leaking when a VRF loopback is received

bgpd: update route leaking when a VRF loopback is received

At bgpd startup, VRF instances are sent from zebra before the
interfaces. When importing a l3vpn prefix from another local VRF
instance, the interfaces are not known yet. The prefix nexthop interface
cannot be set to the loopback or the VRF interface, which causes setting
invalid routes in zebra.

Update route leaking when the loopback or a VRF interface is received
from zebra.

At a VRF interface deletion, zebra voluntarily sends a
ZEBRA_INTERFACE_ADD message to move it to VRF_DEFAULT. Do not update if
such a message is received. VRF destruction will destroy all the related
routes without adding codes.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>