[S390] Switch etr from tasklet to workqueue.

The clock synchronization of the ETR code requires an smp_call_function
to synchronize all cpus. Calling smp_call_function from a tasklet is
illegal. Replace the tasklet with a job on the global workqueue.
ETR work is rare and can be postponed to a be done by a kernel thread.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>