[S390] dasd: fix panic caused by alias device offline

When an alias device is set offline while it is in use this may
result in a panic in the cleanup part of the dasd_block_tasklet.
The problem here is that there may exist some ccw requests that were
originally created for the alias device and transferred to the base
device when the alias was set offline. When these request are
cleaned up later, the discipline pointer in the alias device may not
be valid anymore. To fix this use the base device discipline to find
the cleanup function.

Signed-off-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>