bgpd: release manual vpn label on instance deletion

bgpd: release manual vpn label on instance deletion

When a BGP instance with a manually assigned VPN label is deleted, the
label is not released from the Zebra label registry. As a result,
reapplying a configuration with the same manual label leads to VPN
prefix export failures.

For example, with the following configuration:

> router bgp 65000 vrf BLUE
>  address-family ipv4 unicast
>   label vpn export <int>

Release zebra label registry on unconfiguration.

Fixes: d162d5f6f5 ("bgpd: fix hardset l3vpn label available in mpls pool")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
(cherry picked from commit d6363625c35a99933bf60c9cf0b79627b468c9f7)

# Conflicts:
#	bgpd/bgpd.c