bgpd: fix VRF leaking with 'no bgp network import-check'

bgpd: fix VRF leaking with 'no bgp network import-check'

BGP static routes are defined using the network statement, e.g.:

> router bgp XXX
>  address-family ipv4 unicast
>   network 192.168.0.0/24

When "no bgp network import-check" is set, it is impossible to
successfully import the static routes into the BGP VPN table. The prefix
is present in the table but is not marked as valid. This issue applies
regardless of whether or not routes are present in the router's RIB.

Always mark as valid the nexthops of BGP static routes when "no bgp
network import-check" is set.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
(cherry picked from commit 45bf46441a2e0b02650a1d162367c357b220c7b1)