net: dont use __netdev_alloc_skb for bounce buffer

commit a1c7fff7 (net: netdev_alloc_skb() use build_skb()) broke b44 on
some 64bit machines.

It appears b44 and b43 use __netdev_alloc_skb() instead of alloc_skb()
for their bounce buffers.

There is no need to add an extra NET_SKB_PAD reservation for bounce
buffers :

- In TX path, NET_SKB_PAD is useless

- In RX path in b44, we force a copy of incoming frames if
  GFP_DMA allocations were needed.

Reported-and-bisected-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>