net: support __netdev_alloc_frag to always use GFP_DMA

This make it possible to ensure that any clients which use
__netdev_alloc_frag always allocate memory from the DMA zone.

Change-Id: I608939e8d460cf3e6f39748fd5c73561a051f753
Signed-off-by: Liam Mark <lmark@codeaurora.org>