ocfs2: Pass ocfs2_xattr_value_buf into ocfs2_xattr_value_truncate().

The callers of ocfs2_xattr_value_truncate() now pass in
ocfs2_xattr_value_bufs.  These callers are the ones that calculated the
xv location, so they are the right starting point.

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