dccp: Cleanup routines for feature negotiation

This inserts the required de-allocation routines for memory allocated
by feature negotiation in the socket destructors, replacing
dccp_feat_clean() in one instance.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>