Iommu iotlb can be accessed by different cores for performing IO using
multiple virt queues. Add a spinlock to synchronize iotlb accesses.

This could be easily reproduced when using more than 1 pktgen threads
to inject traffic to vdpa simulator.

Fixes: 2c53d0f6("vdpasim: vDPA device simulator")
Cc: stable@vger.kernel.org
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200731073822.13326-1-jasowang@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>