bgpd: fix interface on leaks from redistribute connected

bgpd: fix interface on leaks from redistribute connected

In the target VRF's Routing Information Base (RIB), routes that are
leaked and originate from the 'redistribute connected' command have
their next-hop interface set as the interface from the source VRF.
This prevents the IP address of the connected interface from being
reachable from the target VRF.

> router bgp 5227 vrf r1-cust1
>  address-family ipv4 unicast
>   redistribute connected
>   rd vpn export 10:1
>   rt vpn import 52:100
>   rt vpn export 52:101
>   export vpn
>   import vpn
>  exit-address-family
> exit
> !
> router bgp 5227 vrf r1-cust4
>  address-family ipv4 unicast
>   network 192.0.2.0/24
>   rd vpn export 10:1
>   rt vpn import 52:101
>   rt vpn export 52:100
>   export vpn
>   import vpn
>  exit-address-family
> exit
> !
> vrf r1-cust1
>  ip route 192.0.2.0/24 r1-cust4 nexthop-vrf r1-cust4

Extract from the routing table:
> VRF r1-cust1:
> C>* 172.16.29.0/24 is directly connected, r1-eth4, 00:44:15
> S>* 192.0.2.0/24 [1/0] is directly connected, r1-cust4 (vrf r1-cust4), weight 1, 00:00:30
>
> VRF r1-cust4:
> B>* 172.16.29.0/24 [20/0] is directly connected, r1-eth4 (vrf r1-cust1), weight 1, 00:00:02

In r1-cust4 VRF, the nexthop interface of 172.16.29.0/24 is r1-eth4,
which is unknown in the context. The following ping does not work:

> # tcpdump -lnni r1-cust1 'icmp' &
> # ip vrf exec r1-cust4 ping -c1 -I 192.0.2.1 172.16.29.1
> PING 172.16.29.1 (172.16.29.1) 56(84) bytes of data.
PING 172.16.29.1 (172.16.29.1) from 192.0.2.1 : 56(84) bytes of data.
18:49:20.635638 IP 192.0.2.1 > 172.16.29.1: ICMP echo request, id 15897, seq 1, length 64
18:49:27.113827 IP 192.0.2.1 > 192.0.2.1: ICMP host 172.16.29.1 unreachable, length 92

Fix the issue by setting nh_ifindex to the index of the VRF master
interface of the incoming BGP instance. The result is:

> VRF r1-cust4:
> C>* 192.0.2.0/24 is directly connected, r1-cust5, 00:27:40
> B>* 172.16.29.0/24 [20/0] is directly connected, r1-cust1 (vrf r1-cust1), weight 1, 00:00:08

> # tcpdump -lnni r1-cust1 'icmp' &
> # ping -c1 172.16.29.1 -I 192.0.2.1
> PING 172.16.29.1 (172.16.29.1) from 192.0.2.1 : 56(84) bytes of data.
> 18:48:32.506281 IP 192.0.2.1 > 172.16.29.1: ICMP echo request, id 15870, seq 1, length 64
> 64 bytes from 172.16.29.1: icmp_seq=1 ttl=64 time=0.050 ms
> 18:48:32.506304 IP 172.16.29.1 > 192.0.2.1: ICMP echo reply, id 15870, seq 1, length 64

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