mm/slab: move NUMA-related code to __do_cache_alloc()

To implement slab_alloc_node() independent of NUMA configuration,
move NUMA fallback/alternate allocation code into __do_cache_alloc().

One functional change here is not to check availability of node
when allocating from local node.

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