f2fs: report error of do_checkpoint

do_checkpoint and write_checkpoint can fail due to reasons like triggering
in a readonly fs or encountering IO error of storage device.

So it's better to report such error info to user, let user be aware of
failure of doing checkpoint.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 3e4a60d..79345e7 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -1838,7 +1838,7 @@
 void add_dirty_dir_inode(struct inode *);
 void remove_dirty_inode(struct inode *);
 void sync_dirty_inodes(struct f2fs_sb_info *, enum inode_type);
-void write_checkpoint(struct f2fs_sb_info *, struct cp_control *);
+int write_checkpoint(struct f2fs_sb_info *, struct cp_control *);
 void init_ino_entry_info(struct f2fs_sb_info *);
 int __init create_checkpoint_caches(void);
 void destroy_checkpoint_caches(void);