xen/privcmd: Mark pages as dirty

pages need to be marked as dirty before unpinned it in
unlock_pages() which was oversight. This is fixed now.

Signed-off-by: Souptick Joarder <[e-mail]>
Suggested-by: John Hubbard <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Reviewed-by: Paul Durrant <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Boris Ostrovsky <[e-mail]>
Cc: Paul Durrant <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/privcmd.c

···
 {
 	unsigned int i;
 
-	for (i = 0; i < nr_pages; i++)
+	for (i = 0; i < nr_pages; i++) {
+		if (!PageDirty(pages[i]))
+			set_page_dirty_lock(pages[i]);
 		put_page(pages[i]);
+	}
 }
 
 static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)