NVMe: extract admin queue size

The queue size of the admin queue should be defined as a constant to use it in
multiple places.
NVMe: rfc blk-mq support

Convert the driver to blk mq.

The patch consists of:

 * Initialization of mq data structures.
 * Convert function calls from bio to request data structures.
 * IO queues are split into an admin queue and io queues.
 * bio splits are removed as it should be handled by block layer.
openssd: use request tag and create a separate admin queue.