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>

mm/slub.c

@@ -6224,7 +6224,7 @@ static int __init slab_sysfs_init(void)
 	if (!slab_kset) {
 		mutex_unlock(&slab_mutex);
 		pr_err("Cannot register slab subsystem.\n");
-		return -ENOSYS;
+		return -ENOMEM;
 	}
 
 	slab_state = FULL;