s390/cio: add rescan functionality on channel subsystem
This patch introduces a new rescan sys-interface for channel-subsystem.
The rescan interface allows the user to invoke an evaluation of all
subchannels defined in the I/O configuration.
The new rescan interface can be found at /sys/devices/css0/rescan
and can be triggered by,
echo > /sys/devices/css0/rescan
Writing to this interface triggers subchannel evaluation. The write
request completes only after scan-related work has completed
This user-invoked subchannel evaluation allows manual recovery in error
situations such as:
- restart of device discovery after resolution of temporary device
error
- inconsistent OS view of subchannel state due to missing state-change
interrupts (CRWs)