FROMGIT: f2fs: flush data when enabling checkpoint back

During checkpoint=disable period, f2fs bypasses all the synchronous IOs such as
sync and fsync. So, when enabling it back, we must flush all of them in order
to keep the data persistent. Otherwise, suddern power-cut right after enabling
checkpoint will cause data loss.

Bug: 171063590
Fixes: 4354994f097d ("f2fs: checkpoint disabling")
Cc: stable@vger.kernel.org
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 8d52dbb373579b48f5758dd0cdd2ac0fb4e5be7f git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git

 dev)
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Change-Id: Iaca2d6fc1841fffa8677d5d592732c94241fb3fb