s390/mcck: fix calculation of SIE critical section size

The size of SIE critical section is calculated wrongly
as result of a missed subtraction in commit 0b0ed657
("s390: remove critical section cleanup from entry.S")

Fixes: 0b0ed657 ("s390: remove critical section cleanup from entry.S")
Cc: <stable@vger.kernel.org>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>