libmount: don't hold write fd to mounted device

libmount: don't hold write fd to mounted device

Avoid holding writeable fd to a loop device that is being mounted. In
the hardened configurations (CONFIG_BLK_DEV_WRITE_MOUNTED = n) the
kernel wants to make sure nobody else has the block device writeably
open when mounting so this makes the mount fail.

Reported-by: JunChao Sun <sunjunchao2870@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Christian Brauner <brauner@kernel.org>
(cherry picked from commit 1cde32f323e0970f6c7f35940dcc0aea97b821e5)