decnet: Parse netlink attributes on our own

decnet is the only subsystem left that is relying on the global
netlink attribute buffer rta_buf. It's horrible design and we
want to get rid of it.

This converts all of decnet to do implicit attribute parsing. It
also gets rid of the error prone struct dn_kern_rta.

Yes, the fib_magic() stuff is not pretty.

It's compiled tested but I need someone with appropriate hardware
to test the patch since I don't have access to it.

Cc: linux-decnet-user@lists.sourceforge.net
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>