veth: ignore peer tx_dropped when counting local rx_dropped

When local NET_RX backlog is full due to traffic overrun,
peer veth tx_dropped counter increases. At that time, list
local veth stats, rx_dropped has double value of peer
tx_dropped, even bigger than transmit packets by peer.

In NET_RX softirq process, if any packet drop case happens,
it increases dev's rx_dropped counter and returns NET_RX_DROP.

At veth tx side, it records any error returned from peer netif_rx
into local dev tx_dropped counter.

In veth get stats process, it puts local dev rx_dropped and
peer dev tx_dropped into together as local rx_drpped value.
So that it shows double value of real dropped packets number in
this case.

This patch ignores peer tx_dropped when counting local rx_dropped,
since peer tx_dropped is duplicated to local rx_dropped at most cases.

Signed-off-by: Jiang Lidong <jianglidong3@jd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>