slab: fix the alias count (via sysfs) of slab cache

We mark some slab caches (e.g.  kmem_cache_node) as unmergeable by
setting refcount to -1, and their alias should be 0, not refcount-1, so
correct it here.
Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
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>

mm/slub.c

@@ -4466,7 +4466,7 @@ SLAB_ATTR_RO(ctor);
 
 static ssize_t aliases_show(struct kmem_cache *s, char *buf)
 {
-	return sprintf(buf, "%d\n", s->refcount - 1);
+	return sprintf(buf, "%d\n", s->refcount < 0 ? 0 : s->refcount - 1);
 }
 SLAB_ATTR_RO(aliases);