mlxsw: spectrum_router: Propagate 'struct mlxsw_sp' further

Next patch is going to make RIF deletion more robust by removing
reliance on fragile mlxsw_sp_lower_get(). This is because a netdev is
not necessarily our upper anymore when its IP addresses are flushed.

The inet{,6}addr notification blocks are going to resolve 'struct
mlxsw_sp' using container_of(), but the functions they call still use
mlxsw_sp_lower_get().

As a preparation for the next patch, propagate 'struct mlxsw_sp' down to
the functions called from the notification blocks and remove reliance on
mlxsw_sp_lower_get().

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