[IPV6]: Fix routing by fwmark

Fix mark comparison, also dump the mask to userspace when the mask is
zero, but the mark is not (in which case the mark is dumped, so the
mask is needed to make sense of it).

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>