xen: introduce mark_page_free

xen: introduce mark_page_free

This commit defines a new helper mark_page_free to extract common code,
like following the same cache/TLB coherency policy, between free_heap_pages
and the new function free_staticmem_pages, which will be introduced later.

The PDX compression makes that conversion between the MFN and the page can
be potentially non-trivial. As the function is internal, pass the MFN and
the page. They are both expected to match.

Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>