KVM: s390: Use "int" as return type for kvm_s390_get/set_skeys()

These two functions only return normal integers, so it does not
make sense to declare the return type as "long" here.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230208140105.655814-3-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>