bgpd: Set metric appropriately for the bnc for a v6 LL address

bgpd: Set metric appropriately for the bnc for a v6 LL address

The v6 LL commit 8761cd6ddb5437767625f58c8e9cc3ccda7887ab

incorrectly was setting the metric value to 1 for the underlying
connected interface.  Modify the code to use a metric value of 0
instead of 1 that now represents the actual metric value that
was originally passed up.

This was noticed when the `show bgp ipv4 uni` command was
inserting a `(metric 1)` into output where before it was not.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>