net: unify the pcpu_tstats and br_cpu_netstats as one

They are same, so unify them as one, pcpu_sw_netstats.

Define pcpu_sw_netstat in netdevice.h, remove pcpu_tstats
from if_tunnel and remove br_cpu_netstats from br_private.h

Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>