nvme: fix visibility of "uuid" ns attribute

"uuid" must be invisible if both ns->uuid and ns->nguid are unset,
not if either one is.

Fixes: d934f9848a77 "nvme: provide UUID value to userspace"
Signed-off-by: Martin Wilck <mwilck@suse.com>
[hch: rebased to the nvme-4.15 tree to help resolving a conflict]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/nvme/host/core.c

@@ -2525,7 +2525,7 @@ static umode_t nvme_ns_id_attrs_are_visible(struct kobject *kobj,
 	struct nvme_ns_ids *ids = &dev_to_ns_head(dev)->ids;
 
 	if (a == &dev_attr_uuid.attr) {
-		if (uuid_is_null(&ids->uuid) ||
+		if (uuid_is_null(&ids->uuid) &&
 		    !memchr_inv(ids->nguid, 0, sizeof(ids->nguid)))
 			return 0;
 	}