[S390] dasd: Use the new async framework for autoonlining.

The dasd driver can automatically online detected dasds, which
especially important for finding the root device. Currently,
it will wait for each online operation to finish individually,
which may take long if many dasds need to be onlined. When using
the new async framework, these onlining operations can run in
parallel and presence of the root device is ensured by the fact
that prepare_namespace() waits for all async threads to finish.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Stefan Haberland <stefan.haberland@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>