net: respect GFP_DMA in __netdev_alloc_skb()

Few drivers use GFP_DMA allocations, and netdev_alloc_frag()
doesn't allocate pages in DMA zone.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>