NVMe: Convert to blk-mq

This converts the NVMe driver to a blk-mq request-based driver.

The NVMe driver is currently bio-based and implements queue logic within itself.
By using blk-mq, a lot of these responsibilities can be moved and simplified.

The patch is divided into the following blocks:

 * Per-command data and cmdid have been moved into the struct request field.
   The cmdid_data can be retrieved using blk_mq_rq_to_pdu() and id maintenance
   are now handled by blk-mq through the rq->tag field.

 * The logic for splitting bio's has been moved into the blk-mq layer. The
   driver instead notifies the block layer about limited gap support in SG
   lists.

 * blk-mq handles timeouts and is reimplemented within nvme_timeout(). This both
   includes abort handling and command cancelation.

 * Assignment of nvme queues to CPUs are replaced with the blk-mq version. The
   current blk-mq strategy is to assign the number of mapped queues and CPUs to
   provide synergy, while the nvme driver assign as many nvme hw queues as
   possible. This can be implemented in blk-mq if needed.

 * NVMe queues are merged with the hardware dispatch queue (hctx) structure of
   blk-mq.

 * blk-mq takes care of setup/teardown of nvme queues and guards invalid
   accesses. Therefore, RCU-usage for nvme queues can be removed.

 * IO tracing and accounting are handled by blk-mq and therefore removed.

 * Setup and teardown of nvme queues are now handled by nvme_[init/exit]_hctx.

Contributions in this patch from:

  Sam Bradshaw <sbradshaw@micron.com>
  Jens Axboe <axboe@kernel.dk>
  Keith Busch <keith.busch@intel.com>
  Robert Nelson <rlnelson@google.com>

Acked-by: Keith Busch <keith.busch@intel.com>
Acked-by: Jens Axboe <axboe@fb.com>
Create readme.md

Description of the lightnvm project
lightnvm: An open FTL for open firmware SSDs

LightNVM implements the internal logic an SSD within the host system.
This includes logic such as translation tables for logical to physical
address translation, garbage collection and wear-leveling.

It is designed to be used either standalone or with a LightNVM
compatible firmware. If used standalone, NVM memory can be simulated
by passing timings to the dm target table. If used with a LightNVM
compatible device, the device will be queued upon initialized for the
relevant values.

The last part is still in progress and a fully working prototype will be
presented in upcoming patches.