ipv6: fix per device IP snmp counters

In commit 4ce3c183 (snmp: 64bit ipstats_mib for all arches), I forgot
to change the /proc/net/dev_snmp6/xxx output for IP counters.

percpu array is 64bit per counter but the folding still used the 'long'
variant, and output garbage on 32bit arches.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>