s390/sclp: rework sclp boundary checks

s390/sclp: rework sclp boundary checks

Rework the SCLP boundary check to account for different SCLP commands
(eventually) allowing different boundary sizes.

Signed-off-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20200915194416.107460-3-walling@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>