bgpd: encode properly vpnv6 nexthop

bgpd: encode properly vpnv6 nexthop

This change updates the nexthop attribute length
accordingly to the safi used. Actually, with the
previous commit, the length calculated was not
aligned with the real nexthop length. Such packet
received by remote peer was malformed, and this
was resulting in breaking vpnv6 peering.

Fix this by updating appropriately the real
nexthop length.

Fixes: 35ac9b53f2e2 ("bgpd: fix vpnv6 nexthop encoding")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>