xen: do not merge reserved pages in free_heap_pages()

xen: do not merge reserved pages in free_heap_pages()

The code in free_heap_pages() will try to merge pages with the
successor/predecessor if pages are suitably aligned. So if the pages
reserved are right next to the pages given to the heap allocator,
free_heap_pages() will merge them, and give the reserved pages to heap
allocator accidentally as a result.

So in order to avoid the above scenario, this commit updates free_heap_pages()
to check whether the predecessor and/or successor has PGC_static set,
when trying to merge the about-to-be-freed chunk with the predecessor
and/or successor.

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