[S390] tape: Fix race condition in tape block device driver

Due to incorrect function call sequence it can happen that a tape block
request is finished before the request is taken from the block request queue.

The following sequence leads to that condition:
 * tapeblock_start_request() -> start CCW program
 * Request finishes -> IO interrupt
 * tapeblock_end_request()
 * end_that_request_last()

If blkdev_dequeue_request() has not been called before end_that_request_last(),
a kernel bug is triggered in end_that_request_last() because the request is
still queued. To solve that problem blkdev_dequeue_request() has to be called
before starting the CCW program.

Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>