net: make napi_threaded_poll() aware of sd->defer_list

If we call skb_defer_free_flush() from napi_threaded_poll(),
we can avoid to raise IPI from skb_attempt_defer_free()
when the list becomes too big.

This allows napi_threaded_poll() to rely less on softirqs,
and lowers latency caused by a too big list.

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