ipv6: fix the use of pcpu_tstats in ip6_vti.c

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: 87b6d218f3adb ("tunnel: implement 64 bits statistics")
Cc: Stephen Hemminger <[email protected]>
Cc: Eric Dumazet <[email protected]>
Signed-off-by: Li RongQing <[email protected]>
Signed-off-by: David S. Miller <[email protected]>