uio: fix finding mm index for vma

When finding mm index for vma it looks more flexible that the mm could
be sparse, and both the size of mm and the pgoff of vma could give
correct selection.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Signed-off-by: Hans J. Koch <hjk@hansjkoch.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/uio/uio.c

@@ -587,14 +587,12 @@ static ssize_t uio_write(struct file *filep, const char __user *buf,
 
 static int uio_find_mem_index(struct vm_area_struct *vma)
 {
-	int mi;
 	struct uio_device *idev = vma->vm_private_data;
 
-	for (mi = 0; mi < MAX_UIO_MAPS; mi++) {
-		if (idev->info->mem[mi].size == 0)
+	if (vma->vm_pgoff < MAX_UIO_MAPS) {
+		if (idev->info->mem[vma->vm_pgoff].size == 0)
 			return -1;
-		if (vma->vm_pgoff == mi)
-			return mi;
+		return (int)vma->vm_pgoff;
 	}
 	return -1;
 }