slab: remove useless statement for checking pfmemalloc

Now, virt_to_page(page->s_mem) is same as the page,
because slab use this structure for management.
So remove useless statement.

Acked-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Pekka Enberg <penberg@iki.fi>