KVM: s390: clear kvm_dirty_regs when dropping to user space

We should make sure that all kvm_dirty_regs bits are cleared before dropping
to user space. Until now, some would remain pending.

Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>