bgpd: do allocate vrf label only when necessary

bgpd: do allocate vrf label only when necessary

Today, with the following bgp instance configured, the
local VRF label is allocated even if it is not used.

> router bgp 65500 vrf vrf1
>  address-family ipv4 unicast
>   label vpn export allocation-mode per-nexthop
>   label vpn export auto
>   rd vpn export 444:1
>   rt vpn both 52:100
>   export vpn
>   import vpn

The 'show mpls table' indicates that the 16 label value
is allocated, but never used in the exported prefixes.

> r1# show mpls table
>  Inbound Label  Type  Nexthop         Outbound Label
>  -----------------------------------------------------
>  16             BGP   vrf1            -
>  17             BGP   192.168.255.13  -
>  18             BGP   192.0.2.12      -
>  19             BGP   192.0.2.11      -

Fix this by only allocating new label values when really
used. Consequently, only 3 labels will be allocated instead
of previously 4.

> r1# show mpls table
>  Inbound Label  Type  Nexthop         Outbound Label
>  -----------------------------------------------------
>  16             BGP   192.168.255.13  -
>  17             BGP   192.0.2.11      -
>  18             BGP   192.0.2.12      -

Fixes: 577be36a41be ("bgpd: add support for l3vpn per-nexthop label")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>