r8169: add gro support

- Use napi_gro_receive() and vlan_gro_receive()
- Enable GRO by default

Tested on a RTL8111/8168 adapter

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>