mlxsw: spectrum_router: Return an error for routes added after abort

We currently do not perform accounting in the driver and thus can't
reject routes before resources are exceeded.

However, in order to make users aware of the fact that routes are no
longer offloaded we can return an error for routes configured after the
abort mechanism was triggered.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>