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: c8a6153b ("vduse: Introduce VDUSE - vDPA Device in Userspace")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Message-Id: <20250704125335.1084649-1-anders.roxell@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>