xen/tmem: Use xen_page_to_gfn rather than pfn_to_gfn

xen/tmem: Use xen_page_to_gfn rather than pfn_to_gfn

All the caller of xen_tmem_{get,put}_page have a struct page * in hand
and call pfn_to_gfn for the only benefits of these 2 functions.

Rather than passing the pfn in parameter, pass directly the page and use
directly xen_page_to_gfn.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit a76e3cc32d872acd5104278d21d2084da07c16a8)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>