add page_get_owner_and_reference() related ASSERT()s

add page_get_owner_and_reference() related ASSERT()s

The function shouldn't return NULL after having obtained a reference,
or else the caller won't know to drop it.

Also its result shouldn't be ignored - if calling code is certain that
a page already has a non-zero refcount, it better ASSERT()s so.

Finally this as well as get_page() and put_page() are required to be
available on all architectures - move the declarations to xen/mm.h.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>