[DCCP]: Generalize dccp_v4_send_reset

Renaming it to dccp_send_reset and moving it from the ipv4 specific
code to the core dccp code.

This fixes some bugs in IPV6 where timers would send v4 resets, etc.

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