dmaengine: bcm2835: Avoid GFP_KERNEL in device_prep_slave_sg

The commit af19b7ce ("mmc: bcm2835: Avoid possible races on
data requests") introduces a possible circular locking dependency,
which is triggered by swapping to the sdhost interface.

So instead of reintroduce the race condition again, we could also
avoid this situation by using GFP_NOWAIT for the allocation of the
DMA buffer descriptors.
Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: af19b7ce ("mmc: bcm2835: Avoid possible races on data requests")
Link: http://lists.infradead.org/pipermail/linux-rpi-kernel/2019-March/008615.htmlSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/bcm2835-dma.c

@@ -671,7 +671,7 @@ static struct dma_async_tx_descriptor *bcm2835_dma_prep_slave_sg(
 	d = bcm2835_dma_create_cb_chain(chan, direction, false,
 					info, extra,
 					frames, src, dst, 0, 0,
-					GFP_KERNEL);
+					GFP_NOWAIT);
 	if (!d)
 		return NULL;