bgpd: fix add label support to EVPN AD routes

bgpd: fix add label support to EVPN AD routes

When peering with an EVPN device from other vendor, FRR acting as route
reflector is not able to read nor transmit the label value.

Actually, EVPN AD routes completely ignore the label value in the code,
whereas in some functionalities like evpn-vpws, it is authorised to
carry and propagate label value.

Fix this by handling the label value.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
(cherry picked from commit 1cefe94dd3991e7e606df8da6b59707295218e55)