[NET]: Introduce INET_ECN_set_ce() function

Changes IP_ECN_set_ce() and IP6_ECN_set_ce() to return 0 if the CE
bits could not bet set because none of the ECT bits are set or 1
if the CE bits are already set or have been successfully set.

Introduces INET_ECN_set_ce(skb) to enable CE bits for all supported
protocols.

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