mlxsw: spectrum_router: Add loopback accessors

struct mlxsw_sp_rif is a router-private structure, and therefore
everything related to it is as well: parameters, and derived RIF types
including loopbacks. IPIP module needs access to some details of
loopback interfaces, but exporting all the RIF shebang would create too
large an interface.

So instead export just the bare minimum necessary: accessors for RIF
index and underlay VRF ID.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>