[DECNET]: Add support for fwmark masks in routing rules

Add support for fwmark masks. For compatibility a mask of 0xFFFFFFFF is used
when a mark value != 0 is sent without a mask.

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