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 <hoeppner@linux.vnet.ibm.com>
Signed-off-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>