mm: slab/slub: use page->list consistently instead of page->lru

'struct page' has two list_head fields: 'lru' and 'list'.  Conveniently,
they are unioned together.  This means that code can use them
interchangably, which gets horribly confusing like with this nugget from
slab.c:

>	list_del(&page->lru);
>	if (page->active == cachep->num)
>		list_add(&page->list, &n->slabs_full);

This patch makes the slab and slub code use page->lru universally instead
of mixing ->list and ->lru.

So, the new rule is: page->lru is what the you use if you want to keep
your page on a list.  Don't like the fact that it's not called ->list?
Too bad.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>