ocfs2: Make 'private' into 'object' on ocfs2_extent_tree.

The 'private' pointer was a way to store off xattr values, which don't
live at a set place in the bh.  But the concept of "the object
containing the extent tree" is much more generic.  For an inode it's the
struct ocfs2_dinode, for an xattr value its the value.  Let's save off
the 'object' at all times.  If NULL is passed to
ocfs2_get_extent_tree(), 'object' is set to bh->b_data;

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>