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.internal
Signed-off-by: Jakub Kicinski <kuba@kernel.org>