Make slab_alloc_node() available even when CONFIG_NUMA=n and
make slab_alloc() wrapper of slab_alloc_node().

This is necessary for further cleanup.

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>