[DCCP]: ditch dccp_v[46]_ctl_send_ack

Merging it with its only user: dccp_v[46]_reqsk_send_ack.

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