net-device: move lstats in net_device_read_txrx

dev->lstats is notably used from loopback ndo_start_xmit()
and other virtual drivers.

Per cpu stats updates are dirtying per-cpu data,
but the pointer itself is read-only.

Fixes: 43a71cd6

 ("net-device: reorganize net_device fast path variables")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Coco Li <lixiaoyan@google.com>
Cc: Simon Horman <horms@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>