ide: make drive->dn read only

The IDE core always sets ->dn correctly so changing it is never
required.

Setting it to a different value than assigned by IDE core is very likely
to result in data corruption (due to wrong transfer timings being set on
the controller etc.)
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Tested-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/ide/ide-proc.c

@@ -206,7 +206,7 @@ static int set_xfer_rate (ide_drive_t *drive, int arg)
 ide_devset_rw(current_speed, xfer_rate);
 ide_devset_rw_field(init_speed, init_speed);
 ide_devset_rw_flag(nice1, IDE_DFLAG_NICE1);
-ide_devset_rw_field(number, dn);
+ide_devset_ro_field(number, dn);
 
 static const struct ide_proc_devset ide_generic_settings[] = {
 	IDE_PROC_DEVSET(current_speed, 0, 70),

include/linux/ide.h

@@ -943,6 +943,10 @@ ide_devset_get(_name, _field); \
 ide_devset_set(_name, _field); \
 IDE_DEVSET(_name, DS_SYNC, get_##_name, set_##_name)
 
+#define ide_devset_ro_field(_name, _field) \
+ide_devset_get(_name, _field); \
+IDE_DEVSET(_name, 0, get_##_name, NULL)
+
 #define ide_devset_rw_flag(_name, _field) \
 ide_devset_get_flag(_name, _field); \
 ide_devset_set_flag(_name, _field); \