net: support __alloc_skb to always use GFP_DMA

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

Change-Id: I3b399b8da113e38a050b0b3e81eeaec549253bb9
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>
Signed-off-by: Ashok Raj D <adeenada@codeaurora.org>