f2fs: introduce f2fs_i_links_write with mark_inode_dirty_sync

This patch introduces f2fs_i_links_write() to call mark_inode_dirty_sync() when
changing inode->i_links.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index aa8cb45..02e0522 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -535,6 +535,7 @@
 	}
 
 	clear_nlink(inode);
+	mark_inode_dirty_sync(inode);
 
 	/* truncate all the data during iput */
 	iput(inode);