[DECNET]: Switch to using ctl_paths.

The decnet includes two places to patch. The first one is
the net/decnet table itself, and it is patched just like
other subsystems in the first patch in this series.

The second place is a bit more complex - it is the
net/decnet/conf/xxx entries,. similar to those in
ipv4/devinet.c and ipv6/addrconf.c. This code is made similar
to those in ipv[46].

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>