f2fs: fix to account preflush command for noflush_merge mode

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: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>