swiotlb: skip swiotlb_bounce when orig_addr is zero

After patch ddbd89d ("swiotlb: fix info leak with DMA_FROM_DEVICE"),
swiotlb_bounce will be called in swiotlb_tbl_map_single unconditionally.
This requires that the physical address must be valid, which is not always
true on stable-4.19 or earlier version.
On stable-4.19, swiotlb_alloc_buffer will call swiotlb_tbl_map_single with
orig_addr equal to zero, which cause such a panic:

Unable to handle kernel paging request at virtual address ffffb77a40000000
...
pc : __memcpy+0x100/0x180
lr : swiotlb_bounce+0x74/0x88
...
Call trace:
 __memcpy+0x100/0x180
 swiotlb_tbl_map_single+0x2c8/0x338
 swiotlb_alloc+0xb4/0x198
 __dma_alloc+0x84/0x1d8
 ...

On stable-4.9 and stable-4.14, swiotlb_alloc_coherent wille call map_single
with orig_addr equal to zero, which can cause same panic.

Fix this by skipping swiotlb_bounce when orig_addr is zero.

Fixes: ddbd89d ("swiotlb: fix info leak with DMA_FROM_DEVICE")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/swiotlb.c

@@ -539,7 +539,8 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
 	 * unconditional bounce may prevent leaking swiotlb content (i.e.
 	 * kernel memory) to user-space.
 	 */
-	swiotlb_bounce(orig_addr, tlb_addr, size, DMA_TO_DEVICE);
+	if (orig_addr)
+		swiotlb_bounce(orig_addr, tlb_addr, size, DMA_TO_DEVICE);
 	return tlb_addr;
 }
 EXPORT_SYMBOL_GPL(swiotlb_tbl_map_single);