slab, kmemleak: pass the correct pointer to kmemleak_erase()

In ____cache_alloc(), the variable 'ac' may be changed after
cache_alloc_refill() and the following kmemleak_erase() may get an incorrect
pointer. Update 'ac' after cache_alloc_refill() unconditionally.

See the following URL for the discussion of this patch:

 http://marc.info/?l=linux-kernel&m=125873373124187&w=2

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>