ocfs2: Fix memory overflow in cow_by_page.

In ocfs2_duplicate_clusters_by_page, we calculate map_end
by shifting page_index. But actually in case we meet with
a large offset(say in a i686 box, poff_t is only 32 bits
and page_index=2056240), we will overflow. So change the
type of page_index to loff_t.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>