sfc: change ARFS expiry mechanism
The old rfs_filters_added method for determining the quota could potentially
allow the NIC to become filled with old filters, which never get tested for
expiry. Instead, explicitly make expiry check work depend on the number of
filters installed, and don't count checking slots without filters in as
doing work. This guarantees that each filter will be checked for expiry at
least once every thirty seconds (assuming the channel to which it belongs is
NAPI polling actively) regardless of fill level.
Signed-off-by: Edward Cree <ecree@solarflare.com>
Tested-by: David Ahern <dahern@digitalocean.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>