vdpa_sim: init iommu lock

The patch adding the iommu lock did not initialize it.
The struct is zero-initialized so this is mostly a problem
when using lockdep.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Cc: Max Gurtovoy <maxg@mellanox.com>
Fixes: 0ea9ee4 ("vdpasim: protect concurrent access to iommu iotlb")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -358,6 +358,7 @@ static struct vdpasim *vdpasim_create(void)
 
 	INIT_WORK(&vdpasim->work, vdpasim_work);
 	spin_lock_init(&vdpasim->lock);
+	spin_lock_init(&vdpasim->iommu_lock);
 
 	dev = &vdpasim->vdpa.dev;
 	dev->coherent_dma_mask = DMA_BIT_MASK(64);