KVM: s390: vsie: stop using "struct page" for vsie page

Now that we no longer use page->index and the page refcount explicitly,
let's avoid messing with "struct page" completely.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Tested-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Message-ID: <20250107154344.1003072-5-david@redhat.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>