[DCCP]: Check if already in the CLOSING state in dccp_rcv_closereq

It is possible to receive more than one CLOSEREQ packet if the
CLOSE packet sent in response is somehow lost, change the state
to DCCP_CLOSING only on the first CLOSEREQ packet received.

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>