xen: correct error for building p2m list on 32 bits

In xen_rebuild_p2m_list() for large areas of invalid or identity
mapped memory the pmd entries on 32 bit systems are initialized
wrong. Correct this error.

Suggested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>