s390/ap: enable sysfs attribute scans to force AP bus rescan

This patch switches the sysfs attribute /sys/bus/ap/scans
from read-only to read-write. If there is something written
to this attribute, an AP bus rescan is forced. If an AP
bus scan is triggered this way a debug feature entry line
reports this in /sys/kernel/debug/s390dbf/ap/sprintf.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Jakob Naucke <naucke@linux.ibm.com>
Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

drivers/s390/crypto/ap_bus.c

@@ -1472,7 +1472,17 @@ static ssize_t scans_show(struct bus_type *bus, char *buf)
 			 atomic64_read(&ap_scan_bus_count));
 }
 
-static BUS_ATTR_RO(scans);
+static ssize_t scans_store(struct bus_type *bus, const char *buf,
+			   size_t count)
+{
+	AP_DBF_INFO("%s force AP bus rescan\n", __func__);
+
+	ap_bus_force_rescan();
+
+	return count;
+}
+
+static BUS_ATTR_RW(scans);
 
 static ssize_t bindings_show(struct bus_type *bus, char *buf)
 {