[PKT_SCHED]: GRED: Support ECN marking

Adds a new u8 flags in a unused padding area of the netlink
message. Adds ECN marking support to be used instead of dropping
packets immediately.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>