Ocfs2: Fix hole punching to correctly do CoW during cluster zeroing.

Based on the previous patch of optimizing truncate, the bugfix for
refcount trees when punching holes can be fairly easy
and straightforward since most of work we should take into account for
refcounting have been completed already in ocfs2_remove_btree_range().

This patch performs CoW for refcounted extents when a hole being punched
whose start or end offset were in the middle of a cluster, which means
partial zeroing of the cluster will be performed soon.

The patch has been tested fixing the following bug:

http://oss.oracle.com/bugzilla/show_bug.cgi?id=1216



Signed-off-by: Tristan Ye <tristan.ye@oracle.com>
Acked-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>