mm: enforce return value checking on get_page()

mm: enforce return value checking on get_page()

It's hard to imagine a case where an error may legitimately be ignored
here. It's bad enough that in at least one case (set_shadow_status())
the return value was checked only by way of ASSERT()ing.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>