ipv6: separate seq_ops for global & per/device ipv6 statistics

idev has been stored on seq->private. NULL has been stored for global
statistics.

The situation is changed with net namespace. We need to store pointer to
struct net and the only place is seq->private. So, we'll have for
/proc/net/dev_snmp6/* and for /proc/net/snmp6 pointers of two different
types stored in the same field.

This effectively requires to separate seq_ops of these files.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>