mm, slab: moved kmem_cache_alloc_node comment to correct place

After several fixing about kmem_cache_alloc_node(), its comment
was splitted. This patch moved it on top of kmem_cache_alloc_node()
definition.

Signed-off-by: Zhouping Liu <zliu@redhat.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>