slub: Use page variable instead of c->page.

Store the value of c->page to avoid additional fetches
from per cpu data.

Change-Id: I72a5e778ba52c55a440de41af5e5fbed0881a17a
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>