veth: rename pcpu_vstats as pcpu_lstats

struct pcpu_vstats and pcpu_lstats have same members and
usage, and pcpu_lstats is used in many files, so rename
pcpu_vstats as pcpu_lstats to reduce duplicate definition

Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>