s390/dcssblk: correct out of bounds array indexes

Fix a couple of warnings like this:

[linux-4.2-rc7/drivers/s390/block/dcssblk.c:553]:
   (style) Array index 'j' is used before limits check.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>