storvsc: do not assume SG list is continuous when doing bounce buffers

The original patch was made for stable 4.1 and was Acked on 08/22/2017, but for
some reason it never made it to the stable tree.

Change from v1:
Changed comment that this patch is for linux-stable 4.1 and all prior stable
kernels.

storvsc checks the SG list for gaps before passing them to Hyper-v device.
If there are gaps, data is copied to a bounce buffer and a continuous data
buffer is passed to Hyper-V.

The check on gaps assumes SG list is continuous, and not chained. This is
 not always true. Failing the check may result in incorrect I/O data
passed to the Hyper-v device.

This code path is not used post Linux 4.1.

[LL: Backport for 4.1]
Signed-off-by: Long Li <longli@microsoft.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>

drivers/scsi/storvsc_drv.c

@@ -582,17 +582,18 @@ static int do_bounce_buffer(struct scatterlist *sgl, unsigned int sg_count)
 	for (i = 0; i < sg_count; i++) {
 		if (i == 0) {
 			/* make sure 1st one does not have hole */
-			if (sgl[i].offset + sgl[i].length != PAGE_SIZE)
+			if (sgl->offset + sgl->length != PAGE_SIZE)
 				return i;
 		} else if (i == sg_count - 1) {
 			/* make sure last one does not have hole */
-			if (sgl[i].offset != 0)
+			if (sgl->offset != 0)
 				return i;
 		} else {
 			/* make sure no hole in the middle */
-			if (sgl[i].length != PAGE_SIZE || sgl[i].offset != 0)
+			if (sgl->length != PAGE_SIZE || sgl->offset != 0)
 				return i;
 		}
+		sgl = sg_next(sgl);
 	}
 	return -1;
 }