bgpd: Only supply ifindex for a v6 nexthop if LL

bgpd: Only supply ifindex for a v6 nexthop if LL

BGP is calculating a v6 routes nexthop as the nexthop address
+ an ifindex.  The ifindex calculated comes from where we received
the route from as that we have to do this for LL addresses.

But a v6 address that is not a LL we do not need to provide
to zebra for nexthop resolution because a global address
by default can be looked up and resolved appropriately.

Modify the code so that we must have an ifindex for a v6 nexthop
if the address is LL, else don't pass the ifindex down to zebra.

Fixes: #1986
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>