scsi: s390: zfcp: Ensure synchronous unit_add

scsi: s390: zfcp: Ensure synchronous unit_add

Improve the usability of the unit_add sysfs attribute by ensuring that
the associated FCP LUN scan processing is completed synchronously.  This
enables configuration tooling to consistently determine the end of the
scan process to allow for serialization of follow-on actions.

While the scan process associated with unit_add typically completes
synchronously, it is deferred to an asynchronous background process if
unit_add is used before initial remote port scanning has completed.  This
occurs when unit_add is used immediately after setting the associated FCP
device online.

To ensure synchronous unit_add processing, wait for remote port scanning
to complete before initiating the FCP LUN scan.

Cc: stable@vger.kernel.org
Reviewed-by: M Nikhil <nikh1092@linux.ibm.com>
Reviewed-by: Nihar Panda <niharp@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Nihar Panda <niharp@linux.ibm.com>
Link: https://lore.kernel.org/r/20250603182252.2287285-2-niharp@linux.ibm.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>