net: ena: use build_skb() in RX path

This patch converts the RX path to use build_skb() for packets larger
than copybreak (set to 256 by default). This function makes the first
descriptor's page to be the linear part of the sk_buff struct buffer.

Also remove the SKB description from the README since most of it no
longer relevant and the parts that are left don't add information.

Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>