slub: Correct the error code when slab_kset is NULL

The -ENOSYS is inproper when kset_create_and_add call returns a NULL
pointer, the failure more likely is because lacking memory, hence
returning -ENOMEM is better.

Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>