net: ena: Do not waste napi skb cache

By profiling, discovered that ena device driver allocates skb by
build_skb() and frees by napi_skb_cache_put(). Because the driver
does not use napi skb cache in allocation path, napi skb cache is
periodically filled and flushed. This is waste of napi skb cache.

As ena_alloc_skb() is called only in napi, Use napi_build_skb()
and napi_alloc_skb() when allocating skb.

This patch was tested on aws a1.metal instance.

[ jwiedmann.dev@gmail.com: Use napi_alloc_skb() instead of
  netdev_alloc_skb_ip_align() to keep things consistent. ]
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: Shay Agroskin <shayagr@amazon.com>
Link: https://lore.kernel.org/r/YfUAkA9BhyOJRT4B@ip-172-31-19-208.ap-northeast-1.compute.internalSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/amazon/ena/ena_netdev.c

@@ -1400,10 +1400,9 @@ static struct sk_buff *ena_alloc_skb(struct ena_ring *rx_ring, void *first_frag)
 	struct sk_buff *skb;
 
 	if (!first_frag)
-		skb = netdev_alloc_skb_ip_align(rx_ring->netdev,
-						rx_ring->rx_copybreak);
+		skb = napi_alloc_skb(rx_ring->napi, rx_ring->rx_copybreak);
 	else
-		skb = build_skb(first_frag, ENA_PAGE_SIZE);
+		skb = napi_build_skb(first_frag, ENA_PAGE_SIZE);
 
 	if (unlikely(!skb)) {
 		ena_increase_stat(&rx_ring->rx_stats.skb_alloc_fail, 1,