net: move definition of pcpu_lstats to header file

pcpu_lstats is defined in several files, so unify them as one
and move to header file

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>