sfc: null out channel->rps_flow_id after freeing it

If an ef100_net_open() fails, ef100_net_stop() may be called without
 channel->rps_flow_id having been written; thus it may hold the address
 freed by a previous ef100_net_stop()'s call to efx_remove_filters().
 This then causes a double-free when efx_remove_filters() is called
 again, leading to a panic.
To prevent this, after freeing it, overwrite it with NULL.

Fixes: a9dc3d56 ("sfc_ef100: RX filter table management and related gubbins")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/rx_common.c

@@ -849,6 +849,7 @@ void efx_remove_filters(struct efx_nic *efx)
 	efx_for_each_channel(channel, efx) {
 		cancel_delayed_work_sync(&channel->filter_work);
 		kfree(channel->rps_flow_id);
+		channel->rps_flow_id = NULL;
 	}
 #endif
 	down_write(&efx->filter_sem);