ipv6: fix the use of pcpu_tstats in ip6_tunnel

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

Fixes: 87b6d218

 ("tunnel: implement 64 bits statistics")

Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>