mlxsw: spectrum_router: Create loopback RIF during initialization

Up until now RIFs (router interfaces) were created on demand (e.g.,
when an IP address was added to a netdev). However, sometimes the device
needs to be provided with a RIF when one might not be available.

For example, adjacency entries that drop packets need to be programmed
with an egress RIF despite the RIF not being used to forward packets.

Create such a RIF during initialization so that it could be used later
on to support blackhole nexthops.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>