f2fs: retrieve IO write stat from the right place

In the following patch,

    f2fs: split journal cache from curseg cache

journal cache is split from curseg cache. So IO write statistics should be
retrived from journal cache but not curseg->sum_blk. Otherwise, it will
get 0, and the stat is lost.

Signed-off-by: Shuoran Liu <liushuoran@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>