xen/gntdev: update to new mmu_notifier semantic

xen/gntdev: update to new mmu_notifier semantic

Calls to mmu_notifier_invalidate_page() were replaced by calls to
mmu_notifier_invalidate_range() and are now bracketed by calls to
mmu_notifier_invalidate_range_start()/end()

Remove now useless invalidate_page callback.

Signed-off-by: Jérôme Glisse <[email protected]>
Reviewed-by: Boris Ostrovsky <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Roger Pau Monné <[email protected]>
Cc: [email protected] (moderated for non-subscribers)
Cc: Kirill A. Shutemov <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>