um: fix ubd_file_size for read-only files

Made ubd_file_size not request write access. Fixes use of read-only images.
Signed-off-by: Martin Pärtel <martin.partel@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/ubd_kern.c

@@ -514,7 +514,7 @@ static inline int ubd_file_size(struct ubd *ubd_dev, __u64 *size_out)
 		goto out;
 	}
 
-	fd = os_open_file(ubd_dev->file, global_openflags, 0);
+	fd = os_open_file(ubd_dev->file, of_read(OPENFLAGS()), 0);
 	if (fd < 0)
 		return fd;