[HACK] fs/super.c: sync ro remount after blocking writers

[Christoph Hellwig]

On Thu, Jan 30, 2014 at 09:26:54AM -0600, Andrew Ruder wrote:
> Move sync_filesystem() after sb_prepare_remount_readonly().  If writers
> sneak in anywhere from sync_filesystem() to sb_prepare_remount_readonly()
> it can cause inodes to be dirtied and writeback to occur well after
> sys_mount() has completely successfully.
> 
> This was spotted by corrupted ubifs filesystems on reboot, but appears
> that it can cause issues with any filesystem using writeback.

>From the link that Richard posted it seems like you have a testcase.
Can you please integrate it into xfstests so that we can properly
regression test for this issue from now on?