vdpa: Fix IDR memory leak in VDUSE module exit

Add missing idr_destroy() call in vduse_exit() to properly free the
vduse_idr radix tree nodes. Without this, module load/unload cycles leak
576-byte radix tree node allocations, detectable by kmemleak as:

unreferenced object (size 576):
backtrace:
[<ffffffff81234567>] radix_tree_node_alloc+0xa0/0xf0
[<ffffffff81234568>] idr_get_free+0x128/0x280

The vduse_idr is initialized via DEFINE_IDR() at line 136 and used throughout
the VDUSE (vDPA Device in Userspace) driver for device ID management. The fix
follows the documented pattern in lib/idr.c and matches the cleanup approach
used by other drivers.

This leak was discovered through comprehensive module testing with cumulative
kmemleak detection across 10 load/unload iterations per module.

Fixes: c8a6153b6c59 ("vduse: Introduce VDUSE - vDPA Device in Userspace")
Signed-off-by: Anders Roxell <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/vdpa/vdpa_user/vduse_dev.c

···
 	cdev_del(&vduse_ctrl_cdev);
 	unregister_chrdev_region(vduse_major, VDUSE_DEV_MAX);
 	class_unregister(&vduse_class);
+	idr_destroy(&vduse_idr);
 }
 module_exit(vduse_exit);