tree: add namespace chrdev name to nvme_ns

tree: add namespace chrdev name to nvme_ns

Namespace device node is initialized into two different nodes: block
device and character device which is a generic device used when block
device is not available due to initialization failure in the kernel.

This can be directly retrieved from the `struct nvme_ns` to print out
the generic node in somewhere like `nvme list` command in nvme-cli.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>