spi: Ensure that sg_table won't be used after being freed

[ Upstream commit 8e9204c ]

SPI code checks for non-zero sgt->orig_nents to determine if the buffer
has been DMA-mapped. Ensure that sg_table is really zeroed after free to
avoid potential NULL pointer dereference if the given SPI xfer object is
reused again without being DMA-mapped.

Fixes: 0c17ba7 ("spi: Fix cache corruption due to DMA/PIO overlap")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20220930113408.19720-1-m.szyprowski@samsung.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/spi/spi.c

@@ -1097,6 +1097,8 @@ void spi_unmap_buf(struct spi_controller *ctlr, struct device *dev,
 	if (sgt->orig_nents) {
 		dma_unmap_sg(dev, sgt->sgl, sgt->orig_nents, dir);
 		sg_free_table(sgt);
+		sgt->orig_nents = 0;
+		sgt->nents = 0;
 	}
 }