ospf6d: Set loopback interface cost to 0

ospf6d: Set loopback interface cost to 0

https://www.rfc-editor.org/rfc/rfc5340.html#page-37 states:

If the interface type is point-
      to-multipoint or the interface is in the state Loopback, the
      global scope IPv6 addresses associated with the interface (if any)
      are copied into the intra-area-prefix-LSA with the PrefixOptions
      LA-bit set, the PrefixLength set to 128, and the metric set to 0.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>