f2fs: use ihold

Use the following helper function committed by Al.

commit 7de9c6ee
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Sat Oct 23 11:11:40 2010 -0400

    new helper: ihold()

...

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>