dmaengine: ioat: Decreasing allocation chunk size 2M->512K

requreing kmalloc of 2M high chance to fail in
fragmented memory.
IOAT ring requires 64k * 64B memory
which will be achived by 512k * 8 allocation
instead of 2M * 2.

Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Leonid Ravich <Leonid.Ravich@emc.com>
Link: https://lore.kernel.org/r/20200416170628.16196-2-leonid.ravich@dell.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>