hv_balloon: simplify hv_online_page()/hv_page_online_one()

Instead of doing pfn_to_page() and continuosly casting page to unsigned
long just cache the pfn of the page with page_to_pfn().

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>