[S390] vmlogrdr: purge after recording is switched off

If automatic purge is enabled for a vmlogrdr device, old records are
purged before an IUCV recording service is switched on or off. If z/VM
generates a large number of records between purging and switching the
recording service off, these records remain queued, and may have a
negative performance impact on the z/VM system. To avoid this problem,
we need to purge the records after recording is switched off.

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