ospfd: Respect loopback's cost that is set and set loopback costs to 0

ospfd: Respect loopback's cost that is set and set loopback costs to 0

When setting an loopback's cost, set the value to 0, unless the operator
has assigned a value for the loopback's cost.

RFC states:

If the state of the interface is Loopback, add a Type 3
link (stub network) as long as this is not an interface
to an unnumbered point-to-point network. The Link ID
should be set to the IP interface address, the Link Data
set to the mask 0xffffffff (indicating a host route),
and the cost set to 0.

FRR is going to allow this to be overridden if the operator specifically
sets a value too.

Fixes: #13472
Signed-off-by: Donald Sharp <sharpd@nvidia.com>