sfc: Implement hardware acceleration of RFS

Use the existing filter management functions to insert TCP/IPv4 and
UDP/IPv4 4-tuple filters for Receive Flow Steering.

For each channel, track how many RFS filters are being added during
processing of received packets and scan the corresponding number of
table entries for filters that may be reclaimed.  Do this in batches
to reduce lock overhead.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>