mlxsw: spectrum_router: Handle down of tunnel underlay

When the bound device of a tunnel device is down, encapsulated packets
are not egressed anymore, but tunnel decap still works. Extend
mlxsw_sp_nexthop_rif_update() to take IFF_UP into consideration when
deciding whether a given next hop should be offloaded.

Because the new logic was added to mlxsw_sp_nexthop_rif_update(), this
fixes the case where a newly-added tunnel has a down bound device, which
would previously be fully offloaded. Now the down state of the bound
device is noted and next hops forwarding to such tunnel are not
offloaded.

In addition to that, notice NETDEV_UP and NETDEV_DOWN of a bound device
to force refresh of tunnel encap route offloads.

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>