NVMe: Rework ioctls

Remove the special-purpose IDENTIFY, GET_RANGE_TYPE, DOWNLOAD_FIRMWARE
and ACTIVATE_FIRMWARE commands.  Replace them with a generic ADMIN_CMD
ioctl that can submit any admin command.

Add a new ID ioctl that returns the namespace ID of the queried device.
It corresponds to the SCSI Idlun ioctl.

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