ovl: sync dirty data when remounting to ro mode

mainline inclusion
from mainline-v5.8-rc1
commit 399c109d
category: bugfix
bugzilla: 186580, https://gitee.com/openeuler/kernel/issues/I52LVN


CVE: NA

--------------------------------

sync_filesystem() does not sync dirty data for readonly filesystem during
umount, so before changing to readonly filesystem we should sync dirty data
for data integrity.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Zheng Liang <zhengliang6@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>