sfc: Feed GRO result into RX allocation policy and interrupt moderation

When GRO is successfully merging received packets, we should allocate
raw page buffers rather than skbs that will be discarded by GRO.
Otherwise, we should allocate skbs.

GRO also benefits from higher interrupt moderation, so increase the
score for mergeable RX packets.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>