KVM: s390: don't use kvm lock in interrupt injection code

The kvm lock protects us against vcpus going away, but they only go
away when the virtual machine is shut down. We don't need this
mutex here, so let's get rid of it.

Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-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>