bgpd: fix label lost when vrf loopback comes back

bgpd: fix label lost when vrf loopback comes back

VRF-label association drops when the VRF loopback goes down, however, it
does not return once the interface is enabled again.

Logs show that when VRF loopback goes down, a label drop message is sent
to zebra and immediately resent label installation to zebra, trigged by
"vpn_leak_postchange_all()":
2024/07/16 13:26:29 BGP: [RVJ1J-J2T22] ifp down r1-cust1 vrf id 7
2024/07/16 13:26:29 BGP: [WA2QY-06STJ] vpn_leak_zebra_vrf_label_withdraw: deleting label for vrf VRF r1-cust1 (id=7)
2024/07/16 13:26:30 BGP: [S82AC-6YAC8] vpn_leak_zebra_vrf_label_update: vrf VRF r1-cust1: afi IPv4: setting label 80 for vrf id 7

Since the interface is down, the netlink message is not send to kernel.
Once the interface comes back, zebra ignore the installation assuming
the label is already seen.

To fix this, add a check for the interface status before attempting to
reinstall the label.

Signed-off-by: Loïc Sang <loic.sang@6wind.com>