[S390] cio: introduce purge function for /proc/cio_ignore

Allow users to remove blacklisted ccw devices by using the
/proc/cio_ignore interface:

  echo purge > /proc/cio_ignore

will remove all devices which are offline and blacklisted.

Signed-off-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>