bgpd: fix label in adj-rib-out

bgpd: fix label in adj-rib-out

After modifying the "label vpn export value", the vpn label information
of the VRF is not updated to the peers.

For example, the 192.168.0.0/24 prefix is announced to the peer with a
label value of 222.

> router bgp 65500
> [..]
>  neighbor 192.0.2.2 remote-as 65501
>  address-family ipv4-vpn
>   neighbor 192.0.2.2 activate
>  exit-address-family
> exit
> router bgp 65500 vrf vrf2
>  address-family ipv4 unicast
>   network 192.168.0.0/24
>   label vpn export 222
>   rd vpn export 444:444
>   rt vpn both 53:100
>   export vpn
>   import vpn
>  exit-address-family

Changing the label with "label vpn export" does not update the label
value to the peer unless the BGP sessions is re-established.

No labels are stored are stored struct bgp_adj_out so that it is
impossible to compare the current value with the previous value
in adj-RIB-out.

Reference the bgp_labels pointer in struct bgp_adj_out and compare the
values when updating adj-RIB-out.

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