f2fs: introduce f2fs_commit_super

This patch introduces f2fs_commit_super to write updated superblock.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -1535,6 +1535,7 @@ static inline int f2fs_add_link(struct dentry *dentry, struct inode *inode)
 /*
  * super.c
  */
+int f2fs_commit_super(struct f2fs_sb_info *);
 int f2fs_sync_fs(struct super_block *, int);
 extern __printf(3, 4)
 void f2fs_msg(struct super_block *, const char *, const char *, ...);

fs/f2fs/super.c

@@ -966,6 +966,30 @@ static int read_raw_super_block(struct super_block *sb,
 	return 0;
 }
 
+int f2fs_commit_super(struct f2fs_sb_info *sbi)
+{
+	struct buffer_head *sbh = sbi->raw_super_buf;
+	sector_t block = sbh->b_blocknr;
+	int err;
+
+	/* write back-up superblock first */
+	sbh->b_blocknr = block ? 0 : 1;
+	mark_buffer_dirty(sbh);
+	err = sync_dirty_buffer(sbh);
+
+	sbh->b_blocknr = block;
+	if (err)
+		goto out;
+
+	/* 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;
+}
+
 static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 {
 	struct f2fs_sb_info *sbi;