s390/cio: avoid excessive path-verification requests

Currently when removing a device from cio_ignore list, we trigger a
path-verification for all the subchannels available in the system. This
could lead to path-verification requests on subchannels with an online
device, which could cause unwanted delay. Instead of all the
subchannels, trigger the path-verifications to those without an online
device.

Reported-by: Julian Wiedmann <jwi@linux.ibm.com>
Fixes: 2297791c ("s390/cio: dont unregister subchannel from child-drivers")
Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>