sfc: limit ARFS workitems in flight per channel

A misconfigured system (e.g. with all interrupts affinitised to all CPUs)
 may produce a storm of ARFS steering events.  With the existing sfc ARFS
 implementation, that could create a backlog of workitems that grinds the
 system to a halt.  To prevent this, limit the number of workitems that
 may be in flight for a given SFC device to 8 (EFX_RPS_MAX_IN_FLIGHT), and
 return EBUSY from our ndo_rx_flow_steer method if the limit is reached.
Given this limit, also store the workitems in an array of slots within the
 struct efx_nic, rather than dynamically allocating for each request.
The limit should not negatively impact performance, because it is only
 likely to be hit in cases where ARFS will be ineffective anyway.

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