dm ioctl: constify ioctl lookup table

Constify the lookup table for device-mapper ioctls so that it is placed
in .rodata.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-ioctl.c

@@ -1629,7 +1629,7 @@ static int target_message(struct file *filp, struct dm_ioctl *param, size_t para
  *---------------------------------------------------------------*/
 static ioctl_fn lookup_ioctl(unsigned int cmd, int *ioctl_flags)
 {
-	static struct {
+	static const struct {
 		int cmd;
 		int flags;
 		ioctl_fn fn;