mm: add fields for compound destructor and order into struct page

Currently, we use lru.next/lru.prev plus cast to access or set
destructor and order of compound page.

Let's replace it with explicit fields in struct page.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>