virtio_net: introduce TX timeout watchdog

This implements ndo_tx_timeout handler and put this into stats. When
there is something wrong to send out packets, we could notice tx timeout
events and total timeout counter.

We have suffered send timeout issues due to the backends hung. With this,
we can find the details, and collect the counters by monitor systems.

Signed-off-by: Tony Lu <tony.ly@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>