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: NShuoran Liu <liushuoran@huawei.com>
Reviewed-by: NChao Yu <chao@kernel.org>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -1470,7 +1470,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 	seg_i = CURSEG_I(sbi, CURSEG_HOT_NODE);
 	if (__exist_node_summaries(sbi))
 		sbi->kbytes_written =
-			le64_to_cpu(seg_i->sum_blk->journal.info.kbytes_written);
+			le64_to_cpu(seg_i->journal->info.kbytes_written);
 
 	build_gc_manager(sbi);