virtio_net: Add a lock for per queue RX coalesce

Once the RTNL locking around the control buffer is removed there can be
contention on the per queue RX interrupt coalescing data. Use a mutex
per queue. A mutex is required because virtnet_send_command can sleep.

Signed-off-by: Daniel Jurgens <danielj@nvidia.com>
Reviewed-by: Heng Qi <hengqi@linux.alibaba.com>
Tested-by: Heng Qi <hengqi@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>