We have commands reserved for internal use.

This is laying the groundwork for removing the internal
queue of commands from the driver so that the locks that
protect that queue may be removed.
Reviewed-by: NScott Teel <scott.teel@pmcs.com>
Signed-off-by: NDon Brace <don.brace@pmcs.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>