mm/slub.c: freelist is ensured to be NULL when new_slab() fails

mm/slub.c: freelist is ensured to be NULL when new_slab() fails

new_slab_objects() will return immediately if freelist is not NULL.

         if (freelist)
                 return freelist;

One more assignment operation could be avoided.

Link: http://lkml.kernel.org/r/20181229062512.30469-1-rocking@whu.edu.cn
Signed-off-by: Peng Wang <rocking@whu.edu.cn>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>