[ Upstream commit a8075dc484cf10ebdb07bee2b17322fb0a846309 ]

Previously, we only account preflush command for flush_merge mode,
so for noflush_merge mode, we can not know in-flight preflush
command count, fix it.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>