nvme: use nvme_disk_is_ns_head helper

Use nvme_disk_is_ns_head helper instead of check fops directly,
and also drop CONFIG_NVME_MULTIPATH check.

Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>