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

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.

CRs-Fixed: 627559
Change-Id:
Ib417b59d39210aab2de4e5ae48b18129e8bc3e26
Cc: Artem Bityutskiy
<dedekind1@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc:
Alexander Viro <viro@zeniv.linux.org.uk>
CC: Richard Weinberger
<richard@nod.at>
Co-authored-by: Richard Weinberger
<richard@nod.at>
Signed-off-by: Andrew Ruder
<andrew.ruder@elecsyscorp.com>
Signed-off-by: Al Viro
<viro@zeniv.linux.org.uk>
Git-commit: 807612d
Git-repo:
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Signed-off-by:
Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by:
franciscofranco <franciscofranco.1990@gmail.com>

idea is taken here -
https://github.com/franciscofranco/mako/commit/1dc2fdfddfbb1b5fed40c07a3239cdc208441e8c