bgpd: fix bgp node created at withdraw event

bgpd: fix bgp node created at withdraw event

The prefixes unexportation triggers an attempt to create
the VPN prefix node if that prefix was not already present.

For instance, if a given prefix is not exported because of
a route-map filtering, the withdraw process will try to
create the node with the 'bgp_afi_node_get()' command.

Fix this by replacing this call by the 'bgp_safi_node_lookup()'
function.

Fixes: ddb5b4880ba8 ("bgpd: vpn-vrf route leaking")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>