dpif-netdev: Flush offload rules upon port deletion.

dpif-netdev: Flush offload rules upon port deletion.

When a port is deleted, flow deletion requests are posted, and the netdev
is removed from offload netdevs map. Following flow deletion handling may
be done after the netdev has already been removed from the offload
netdevs map, so the HW rule is not removed and the data object is not
freed (memory leak). Flush offload rules upon port deletion, and disable
pending handling of offloads to fix it.

Signed-off-by: Eli Britstein <elibr@nvidia.com>
Reviewed-by: Gaetan Rivet <gaetanr@nvidia.com>
Acked-by: Emma Finn <emma.finn@intel.com>
Tested-by: Emma Finn <emma.finn@intel.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>