bgpd: fix VRF leaking with 'network import-check' (1/4)

bgpd: fix VRF leaking with 'network import-check' (1/4)

If 'bgp network import-check' is defined on the source BGP session,
prefixes that are defined with the network command cannot be leaked to
the other VRFs BGP table even if they are present in the origin VRF RIB.

Always validate the nexthop of BGP static routes (i.e. defined with the
network statement) if 'network import-check' is defined on the source
BGP session and the prefix is present in source RIB.

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