virtio_net: suppress cpu stall when free_unused_bufs

For multi-queue and large ring-size use case, the following error
occurred when free_unused_bufs:
rcu: INFO: rcu_sched self-detected stall on CPU.

Fixes: 986a4f4d

 ("virtio_net: multiqueue support")
Signed-off-by: Wenliang Wang <wangwenliang.1995@bytedance.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>