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/checkpoint.c b/fs/f2fs/checkpoint.c
index 669f64f..01aae09 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -1052,7 +1052,7 @@
 	if (sb->s_bdev->bd_part)
 		kbytes_written += BD_PART_WRITTEN(sbi);
 
-	seg_i->sum_blk->info.kbytes_written = cpu_to_le64(kbytes_written);
+	seg_i->sum_blk->journal.info.kbytes_written = cpu_to_le64(kbytes_written);
 
 	if (__remain_node_summaries(cpc->reason)) {
 		write_node_summaries(sbi, start_blk);