[S390] tape block: complete request with correct locking

__blk_end_request must be called with request queue lock held. We need to use
blk_end_request rather than  __blk_end_request.

Signed-off-by: Frank Munzert <munzert@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>