ip6_tunnel: add optional fwmark inherit

Add IP6_TNL_F_USE_ORIG_FWMARK to ip6_tunnel, so that ip6_tnl_xmit2()
makes a route lookup taking into account skb->fwmark and doesnt cache
lookup result.

This permits more flexibility in policies and firewall setups.

To setup such a tunnel, "fwmark inherit" option should be added to "ip
-f inet6 tunnel" command.

Reported-by: Anders Franzen <Anders.Franzen@ericsson.com>
CC: Hans Schillström <hans.schillstrom@ericsson.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>