nvme: implement mq_ops->commit_rqs() hook

Split the command submission and the SQ doorbell ring, and add the
doorbell ring as our ->commit_rqs() hook. This allows a list of
requests to be issued, with nvme only writing the SQ update when
it's necessary. This is more efficient if we have lists of requests
to issue, particularly on virtualized hardware, where writing the
SQ doorbell is more expensive than on real hardware. For those cases,
performance increases of 2-3x have been observed.

The use case for this is plugged IO, where blk-mq flushes a batch of
requests at the time.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>