f2fs: refactor f2fs_commit_super

Previously, f2fs_commit_super hacks the bh->blocknr to write the broken
alternate superblock.
Instead of it, we should use the correct logic to retrieve its buffer head
with locking it appropriately.
Reviewed-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -1099,27 +1099,35 @@ static int read_raw_super_block(struct super_block *sb,
 int f2fs_commit_super(struct f2fs_sb_info *sbi, bool recover)
 {
 	struct buffer_head *sbh = sbi->raw_super_buf;
-	sector_t block = sbh->b_blocknr;
+	struct buffer_head *bh;
 	int err;
 
 	/* write back-up superblock first */
-	sbh->b_blocknr = block ? 0 : 1;
-	mark_buffer_dirty(sbh);
-	err = sync_dirty_buffer(sbh);
+	bh = sb_getblk(sbi->sb, sbh->b_blocknr ? 0 : 1);
+	if (!bh)
+		return -EIO;
 
-	sbh->b_blocknr = block;
+	lock_buffer(bh);
+	memcpy(bh->b_data, sbh->b_data, sbh->b_size);
+	WARN_ON(sbh->b_size != F2FS_BLKSIZE);
+	set_buffer_uptodate(bh);
+	set_buffer_dirty(bh);
+	unlock_buffer(bh);
+
+	/* it's rare case, we can do fua all the time */
+	err = __sync_dirty_buffer(bh, WRITE_FLUSH_FUA);
+	brelse(bh);
 
 	/* if we are in recovery path, skip writing valid superblock */
 	if (recover || err)
-		goto out;
+		return err;
 
 	/* write current valid superblock */
-	mark_buffer_dirty(sbh);
-	err = sync_dirty_buffer(sbh);
-out:
-	clear_buffer_write_io_error(sbh);
-	set_buffer_uptodate(sbh);
-	return err;
+	lock_buffer(sbh);
+	set_buffer_dirty(sbh);
+	unlock_buffer(sbh);
+
+	return __sync_dirty_buffer(sbh, WRITE_FLUSH_FUA);
 }
 
 static int f2fs_fill_super(struct super_block *sb, void *data, int silent)