Use the generic dma remapping allocator instead of open coding it.
This also avoids setting up page tables from irq context which is
generally dangerous and uses the atomic pool instead.

Note that this changes the kernel virtual address at which the
dma coherent memory is mapped from the DVMA_VADDR region to the general
vmalloc pool.  I could not find any indication that this matters
for the hardware.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Andreas Larsson <andreas@gaisler.com>
Acked-by: David S. Miller <davem@davemloft.net>