Hibernate: Replace unnecessary evaluation of pfn_to_page()

Replace one evaluation of pfn_to_page() in copy_data_pages() with
the value of a local variable containing the right number already.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Len Brown <len.brown@intel.com>