ipv6: fix the use of pcpu_tstats in ip6_vti.c

when read/write the 64bit data, the correct lock should be hold.
and we can use the generic vti6_get_stats to return stats, and
not define a new one in ip6_vti.c

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>