NVMe: Add a character device for each nvme device

Registers a miscellaneous device for each nvme controller probed. This
creates character device files as /dev/nvmeN, where N is the device
instance, and supports nvme admin ioctl commands so devices without
namespaces can be managed.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>