r8169: use netdev_alloc_skb

Use netdev_alloc_skb and remove the useless sk_buff * argument of
rtl8169_alloc_rx_skb.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>