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: d934f984 "nvme: provide UUID value to userspace"
Signed-off-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>