f2fs: make clean inodes when flushing inode page

This patch tries to make more clean inodes when flushing dirty inodes in
checkpoint.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
index bfa512d..7b5e402 100644
--- a/fs/f2fs/inode.c
+++ b/fs/f2fs/inode.c
@@ -339,7 +339,7 @@ int f2fs_write_inode(struct inode *inode, struct writeback_control *wbc)
 	 * We need to balance fs here to prevent from producing dirty node pages
 	 * during the urgent cleaning time when runing out of free sections.
 	 */
-	if (update_inode_page(inode))
+	if (update_inode_page(inode) && wbc && wbc->nr_to_write)
 		f2fs_balance_fs(sbi, true);
 	return 0;
 }