bgpd: fix prefix VRF leaking with 'network import-check' (4/5)

bgpd: fix prefix VRF leaking with 'network import-check' (4/5)

If 'network import-check' is defined on the source BGP session, prefixes
that are stated in the network command cannot be leaked to the other
VRFs BGP table even if they are present in the origin VRF RIB if the
'rt import' statement is defined after the 'network <prefix>' ones.

When a prefix nexthop is updated, update the prefix route leaking. The
current state of nexthop validation is now stored in the attributes of
the bgp path info. Attributes are compared with the previous ones at
route leaking update so that a nexthop validation change now triggers
the update of destination VRF BGP table.

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