"if not XX else" statements are confusing.

Replace two "if not XX else" statements by "if XX else" to prepare next
commits. The patch is only cosmetic.

Signed-off-by: Louis Scalbert <[email protected]>

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
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 <[email protected]>

The following configuration creates an infinite routing leaking loop
because 'rt vpn both' parameters are the same in both VRFs.

> router bgp 5227 vrf r1-cust4
>    no bgp network import-check
>    bgp router-id 192.168.1.1
>    address-family ipv4 unicast
>      network 28.0.0.0/24
>      rd vpn export 10:12
>      rt vpn both 52:100
>      import vpn
>      export vpn
>    exit-address-family
> !
> router bgp 5227 vrf r1-cust5
>    no bgp network import-check
>    bgp router id 192.168.1.1
>    address-family ipv4 unicast
>      network 29.0.0.0/24
>      rd vpn export 10:13
>      rt vpn both 52:100
>      import vpn
>      export vpn
>    exit-address-family

The previous commit has added a routing leak update when a nexthop
update is received from zebra. It indirectly calls
bgp_find_or_add_nexthop() in which a static route triggers a nexthop
cache entry registration that triggers a nexthop update from zebra.

Do not register again the nexthop cache entry if the BGP_STATIC_ROUTE is
already set.

Signed-off-by: Louis Scalbert <[email protected]>

Test vpnv4 route leaking with import-check

Signed-off-by: Louis Scalbert <[email protected]>