dma-mapping: clear harmful GFP_* flags in common code

Lift the code from x86 so that we behave consistently.  In the future we
should probably warn if any of these is set.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org> [m68k]