r8169: use 50% less ram for RX ring

Using standard skb allocations in r8169 leads to order-3 allocations (if
PAGE_SIZE=4096), because NIC needs 16383 bytes, and skb overhead makes
this bigger than 16384 -> 32768 bytes per "skb"

Using kmalloc() permits to reduce memory requirements of one r8169 nic
by 4Mbytes. (256 frames * 16Kbytes). This is fine since a hardware bug
requires us to copy incoming frames, so we build real skb when doing
this copy.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>