mm, slab_common: add 'unlikely' to size check of kmalloc_slab()

Size is usually below than KMALLOC_MAX_SIZE.
If we add a 'unlikely' macro, compiler can make better code.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>