[DCCP]: Transition from PARTOPEN to OPEN when receiving DATA packets

Noticed by Andrea Bittau, that provided a patch that was modified to
not transition from RESPOND to OPEN when receiving DATA packets.

Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: David S. Miller <davem@davemloft.net>