Ocfs2: Teach local-mounted ocfs2 to handle unwritten_extents correctly.

Oops, local-mounted of 'ocfs2_fops_no_plocks' is just missing the support
of unwritten_extents/punching-hole due to no func pointer was given correctly
to '.follocate' field.

Signed-off-by: Tristan Ye <tristan.ye@oracle.com>