mm/slab: consolidate includes in the internal mm/slab.h

The #include's are scattered at several places of the file, but it does
not seem this is needed to prevent any include loops (anymore?) so
consolidate them at the top. Also move the misplaced kmem_cache_init()
declaration away from the top.

Acked-by: David Rientjes <rientjes@google.com>
Tested-by: David Rientjes <rientjes@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>