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)

Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>