ipv6: fix the use of pcpu_tstats in ip6_tunnel

commit abb6013cca147ad940b0e9fee260d2d9e93b7018 upstream.

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>
Signed-off-by: Willy Tarreau <w@1wt.eu>