vfio/fsl-mc: prevent underflow in vfio_fsl_mc_mmap()

My static analsysis tool complains that the "index" can be negative.
There are some checks in do_mmap() which try to prevent underflows but
I don't know if they are sufficient for this situation.  Either way,
making "index" unsigned is harmless so let's do it just to be safe.

Fixes: 6724728 ("vfio/fsl-mc: Allow userspace to MMAP fsl-mc device MMIO regions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Diana Craciun <diana.craciun@oss.nxp.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: Dan Carpenter

drivers/vfio/fsl-mc/vfio_fsl_mc.c

@@ -472,7 +472,7 @@ static int vfio_fsl_mc_mmap(void *device_data, struct vm_area_struct *vma)
 {
 	struct vfio_fsl_mc_device *vdev = device_data;
 	struct fsl_mc_device *mc_dev = vdev->mc_dev;
-	int index;
+	unsigned int index;
 
 	index = vma->vm_pgoff >> (VFIO_FSL_MC_OFFSET_SHIFT - PAGE_SHIFT);