compat_ioctl: fix block device compat ioctl regression

The conversion of handlers to compat_blkdev_ioctl accidentally
disabled handling of most ioctl numbers on block devices because
of a typo. Fix the one line to enable it all again.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NJens Axboe <axboe@carl.home.kernel.dk>

block/compat_ioctl.c

@@ -581,7 +581,7 @@ static int compat_blkdev_driver_ioctl(struct inode *inode, struct file *file,
 {
 	int ret;
 
-	switch (arg) {
+	switch (cmd) {
 	case HDIO_GET_UNMASKINTR:
 	case HDIO_GET_MULTCOUNT:
 	case HDIO_GET_KEEPSETTINGS: