[DCCP]: Replace socket with sock for reset sending.

Replace dccp_v(4|6)_ctl_socket with sock to unify a code with TCP/ICMP.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>