net: use napi_consume_skb() in skb_defer_free_flush()

skb_defer_free_flush() runs from softirq context,
we have the opportunity to refill the napi_alloc_cache,
and/or use kmem_cache_free_bulk() when this cache is full.

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