sfc: replace asynchronous filter operations

Instead of having an efx->type->filter_rfs_insert() method, just use
 workitems with a worker function that calls efx->type->filter_insert().
The only user of this is efx_filter_rfs(), which now queues a call to
 efx_filter_rfs_work().
Similarly, efx_filter_rfs_expire() is now a worker function called on a
 new channel->filter_work work_struct, so the method
 efx->type->filter_rfs_expire_one() is no longer called in atomic context.
 We also add a new mutex efx->rps_mutex to protect the RPS state (efx->
 rps_expire_channel, efx->rps_expire_index, and channel->rps_flow_id) so
 that the taking of efx->filter_lock can be moved to
 efx->type->filter_rfs_expire_one().
Thus, all filter table functions are now called in a sleepable context,
 allowing them to use sleeping locks in a future patch.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>