ubi: Fix error for write access

BugLink: http://bugs.launchpad.net/bugs/1768429

commit 78a8dfba upstream.

When opening a device with write access, ubiblock_open returns an error
code. Currently, this error code is -EPERM, but this is not the right
value.

The open function for other block devices returns -EROFS when opening
read-only devices with FMODE_WRITE set. When used with dm-verity, the
veritysetup userspace tool is expecting EROFS, and refuses to use the
ubiblock device.

Use -EROFS for ubiblock as well. As a result, veritysetup accepts the
ubiblock device as valid.

Cc: stable@vger.kernel.org
Fixes: 9d54c8a3 (UBI: R/O block driver on top of UBI volumes)
Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/mtd/ubi/block.c

@@ -244,7 +244,7 @@ static int ubiblock_open(struct block_device *bdev, fmode_t mode)
 	 * in any case.
 	 */
 	if (mode & FMODE_WRITE) {
-		ret = -EPERM;
+		ret = -EROFS;
 		goto out_unlock;
 	}