slab: use 32-bit arithmetic in freelist_randomize()

SLAB doesn't support 4GB+ of objects per slab, therefore randomization
doesn't need size_t.

Link: http://lkml.kernel.org/r/20180305200730.15812-25-adobriyan@gmail.com
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>