vfio/mdev: Constify struct kobj_type

This 'struct kobj_type' is not modified. It is only used in
kobject_init_and_add() which takes a 'const struct kobj_type *ktype'
parameter.

Constifying this structure and moving it to a read-only section,
and this can increase over all security.

```
[Before]
text data bss dec hex filename
2372 600 0 2972 b9c drivers/vfio/mdev/mdev_sysfs.o

[After]
text data bss dec hex filename
2436 568 0 3004 bbc drivers/vfio/mdev/mdev_sysfs.o
```

Signed-off-by: Hongbo Li <[e-mail]>
Reviewed-by: Jason Gunthorpe <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alex Williamson <[e-mail]>

drivers/vfio/mdev/mdev_sysfs.c

···
 	put_device(type->parent->dev);
 }
 
-static struct kobj_type mdev_type_ktype = {
+static const struct kobj_type mdev_type_ktype = {
 	.sysfs_ops	= &mdev_type_sysfs_ops,
 	.release	= mdev_type_release,
 	.default_groups	= mdev_type_groups,