s390/dasd: Remove variable sized array

Dynamic stack allocations are considered bad. Get rid of this one
occurrence and use kstrdup() instead.

Also, set the return codes so that we have only one exit where we can
call kfree().

Signed-off-by: Jan Höppner <hoeppner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>