r8169: Use eth_skb_pad function

Replace rtl_skb_pad with eth_skb_pad since they do the same thing.

Cc: Realtek linux nic maintainers <nic_swsd@realtek.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>