[DCCP]: Initialize icsk_rto in dccp_v4_init_sock

Fixes nasty bug related to the retransmit timer (yeah, DCCP does
retransmits) firing too early.

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