nvme: introduce nvme_disk_is_ns_head helper

We currently rely on gendisk's file operations (fops) to distinguish
between a namespace head (ns_head) and a regular namespace. To enhance
code readability, introduce a helper function.
Additionally, we must ensure that the device is not an ns_head before
calling nvme_get_ns_from_dev(). To enforce this, add a WARN_ON check
within the nvme_get_ns_from_dev().

Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Liu Song <liusong@linux.alibaba.com>
[include fix: https://lore.kernel.org/oe-kbuild-all/202401031943.0N72Tkji-lkp@intel.com/

]
Signed-off-by: Keith Busch <kbusch@kernel.org>