ocfs2: Explain t_is_new in ocfs2_cp_xattr_cluster().

I was unsure of the JOURNAL_ACCESS parameters in
ocfs2_cp_xattr_cluster().  They're based on the function argument
't_is_new', but I couldn't quite figure out how t_is_new mapped to
allocation.  ocfs2_cp_xattr_cluster() actually overwrites the target,
regardless of t_is_new.

Well, I just figured it out.  So I'm adding a big fat comment for those
who come after me.  ocfs2_divide_xattr_cluster() has the same behavior.

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