drivers: net: generalize napi_complete_done()

napi_complete_done() allows to opt-in for gro_flush_timeout,
added back in linux-3.19, commit 3b47d303


("net: gro: add a per device gro flush timer")

This allows for more efficient GRO aggregation without
sacrifying latencies.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>