ipv6: fix the use of pcpu_tstats in sit

when read/write the 64bit data, the correct lock should be hold.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>