s390/scm: use common completion path

Since commit caf7df12 ("block: remove the errors field from struct request")
rq->errors can't be (mis)used by block device drivers to store the error
condition for usage during async completion. Because of that I simply used
async completion only for the non-error paths.

This patch places the error within the private data of struct request and
uses async completion for all paths again.

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