[DECNET] Fix to multiple tables routing

Here is a fix to Patrick McHardy's increase number of routing tables
patch for DECnet. I did just test this and it appears to be working
fine with this patch.

Signed-off-by: Steven Whitehouse <steve@chygwyn.com>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>