s390/cio: fix race condition during online processing

A race condition exists in ccw_device_set_online() that can cause the
online process to fail, leaving the affected device in an inconsistent
state. As a result, subsequent attempts to set that device online fail
with return code ENODEV.

The problem occurs when a path verification request arrives after
a wait for final device state completed, but before the result state
is evaluated.

Fix this by ensuring that the CCW-device lock is held between
determining final state and checking result state.

Note that since:

commit 2297791c ("s390/cio: dont unregister subchannel from child-drivers")

path verification requests are much more likely to occur during boot,
resulting in an increased chance of this race condition occurring.

Fixes: 2297791c ("s390/cio: dont unregister subchannel from child-drivers")
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>