nvme-core: use nvme_ns_head_multipath instead of ns->head->disk

Change the way we check for a multipath nshead so as
to consistently use the same check to assert the same condition.

Signed-off-by: Irvin Cote <irvincoteg@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>