xen/tmem: Use page_to_gfn rather than pfn_to_gfn

xen/tmem: Use 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 page_to_gfn.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>