[ATALK/DECNET]: Use seq_open_private in appletalk and decnet.

These two also perform manual seq_open_private, so patch them both at
once. But unlike ATM code, these already use the seq_release_private,
so I splitted this patch from the previous one.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>