scsi: Restrict user space SCSI device state changes to "running" and "offline"

The ability to modify the SCSI device state was introduced by commit
638127e579a4 ("[PATCH] Fix error handler offline behaviour"; v2.6.12). That
same commit introduced the following device states:

       { SDEV_CREATED, "created" },
       { SDEV_RUNNING, "running" },
       { SDEV_CANCEL,  "cancel"  },
       { SDEV_DEL,     "deleted" },
       { SDEV_QUIESCE, "quiesce" },
       { SDEV_OFFLINE, "offline" },

The SDEV_BLOCK state was introduced later to avoid that an FC cable pull
would immediately result in an I/O error (commit 1094e682310e; "[PATCH]
suspending I/Os to a device"; v2.6.12). That same patch introduced the
ability to set the SDEV_BLOCK state from user space. I'm not sure whether
that ability was introduced on purpose or accidentally.

Since there is agreement that only writing "running" or "offline" into
the SCSI sysfs device state attribute makes sense, restrict sysfs writes
to these values.

This patch makes sure that SDEV_BLOCK is only used for its original
purpose, namely to allow transport drivers and LLDs to block further
.queuecommand() calls while transport layer or adapter recovery is in
progress.

Note: a web search for "/sys/class/scsi_device" AND "device/state"
revealed several storage configuration guides. The instructions I found
in these guides tell users to write the value "running" or "offline" in
the SCSI device state sysfs attribute and no other values.

[mkp: typo]

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: James Smart <james.smart@broadcom.com>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_sysfs.c

@@ -767,8 +767,13 @@ store_state_field(struct device *dev, struct device_attribute *attr,
 			break;
 		}
 	}
-	if (!state)
+	switch (state) {
+	case SDEV_RUNNING:
+	case SDEV_OFFLINE:
+		break;
+	default:
 		return -EINVAL;
+	}
 
 	mutex_lock(&sdev->state_mutex);
 	ret = scsi_device_set_state(sdev, state);