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: a9dc3d5612ce ("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>