f2fs: introduce f2fs_journal struct to wrap journal info

Introduce a new structure f2fs_journal to wrap journal info in struct
f2fs_summary_block for readability.

struct f2fs_journal {
	union {
		__le16 n_nats;
		__le16 n_sits;
	};
	union {
		struct nat_journal nat_j;
		struct sit_journal sit_j;
		struct f2fs_extra_info info;
	};
} __packed;

struct f2fs_summary_block {
	struct f2fs_summary entries[ENTRIES_IN_SUM];
	struct f2fs_journal journal;
	struct summary_footer footer;
} __packed;

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index fa4bbe0..ecee9ef 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1414,7 +1414,7 @@
 	seg_i = CURSEG_I(sbi, CURSEG_HOT_NODE);
 	if (__exist_node_summaries(sbi))
 		sbi->kbytes_written =
-			le64_to_cpu(seg_i->sum_blk->info.kbytes_written);
+			le64_to_cpu(seg_i->sum_blk->journal.info.kbytes_written);
 
 	build_gc_manager(sbi);