[DECNET]: Covert rules to use generic code

This patch converts the DECnet rules code to use the generic
rules system created by Thomas Graf <tgraf@suug.ch>.

Signed-off-by: Steven Whitehouse <steve@chygwyn.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>