s390/dasd: blk-mq conversion

Use new blk-mq interfaces. Use multiple queues and also use the block
layer complete helper that finish the IO on the CPU that initiated it.

Reviewed-by: Jan Hoeppner <[email protected]>
Signed-off-by: Stefan Haberland <[email protected]>
Signed-off-by: Martin Schwidefsky <[email protected]>

Author: stefan-haberland