slub: Eliminate repeated use of c->page through a new page variable

__slab_alloc is full of "c->page" repeats. Lets just use one local variable
named "page" for this. Also avoids the need to a have another variable
called "new".

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>