decnet: Fix disappearing sysctl entries

When decnet is built as a module a simple:
echo 0.0 >/proc/sys/net/decnet/node_address

results in most of the sysctl entries under /proc/sys/net/decnet and
/proc/sys/net/decnet/conf disappearing.

For more details see http://www.spinics.net/lists/netdev/msg226123.html.

This change applies the same workaround used in
net/core/sysctl_net_core.c and net/ipv6/sysctl_net_ipv6.c of creating
a skeleton of decnet sysctl entries before doing anything else.

The problem first appeared in kernel 2.6.27.  The later rewrite of
sysctl in kernel 3.4 restored the previous behavior and eliminated the
need for this workaround.

This patch was heavily inspired by a similar but more complex patch by
Larry Baker.

Reported-by: Larry Baker <baker@usgs.gov>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: David Miller <davem@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>