[S390] cio: avoid memory leak on error in css_alloc_subchannel().

sch->lock has been allocated in cio_validate_subchannel(), it must be
freed if cio_modify() fails.

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