[IPV6] ROUTE: Add support for fwmask in routing rules.

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

Based on patch for net/ipv4/fib_rules.c by Patrick McHardy <kaber@trash.net>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>