KVM: s390: Zero out current VMDB of STSI before including level3 data.

s390 documentation requires words 0 and 10-15 to be reserved and stored as
zeros. As we fill out all other fields, we can memset the full structure.

Signed-off-by: Ekaterina Tumanova <tumanova@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>