s390/scm_block: fix off by one during cluster reservation

We increase the msb_count after we're finished building the request.
That way we can always access the current request via
scmrq->request[msb_count] . But once the request is started we need
to make sure that the array index stays below msb_count.

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