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 <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-ioctl.c

···
  *---------------------------------------------------------------*/
 static ioctl_fn lookup_ioctl(unsigned int cmd, int *ioctl_flags)
 {
-	static struct {
+	static const struct {
 		int cmd;
 		int flags;
 		ioctl_fn fn;