s390/scm_blk: fix request number accounting

If a block device driver cannot fetch all requests from the blocklayer
it's in his responsibility to call the request function at a later time.
Normally this would be done after the next irq for the underlying device
is handled. However in situations where we have no outstanding request
we have to schedule the request function for a later time.

This is determined using an internal counter of requests issued to the
hardware.

In some cases where we give a request back to the block layer unhandled
the number of queued requests was not adjusted. Fix this class of
failures by adjusting queued_requests in all functions used to give
a request back to the block layer.

Reviewed-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>