xen: Fix page <-> pfn conversion on 32 bit systems

xen: Fix page <-> pfn conversion on 32 bit systems

Commit 8a3fda262196e8ff5cc541154fd6ce14611bcb38 (xen: Add Xen specific
page definition) caused a regression in 4.4.

The xen functions to convert between pages and pfns fail due to an
overflow on systems where a physical address may not fit in an
unsigned long (e.g. x86 32 bit PAE systems). Rework the conversion to
avoid overflow. This should also result in simpler object code.

This bug manifested itself as disk corruption with Linux 4.4 when
using blkfront in a Xen HVM x86 32 bit guest with more than 4 GiB of
memory.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Cc: <stable@vger.kernel.org> # 4.4+
Signed-off-by: David Vrabel <david.vrabel@citrix.com>