dccp: Registration routines for changing feature values

Two registration routines, for SP and NN features, are provided by this patch,
replacing a previous routine which was used for both feature types.

These are internal-only routines and therefore start with `__feat_register'.

It further exports the known limits of Sequence Window and Ack Ratio as symbolic
constants.

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>
diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index dafcefd..01332fe 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -202,7 +202,7 @@
 	 * setsockopt(CCIDs-I-want/accept). -acme
 	 */
 	if (likely(ctl_sock_initialized)) {
-		int rc = dccp_feat_init(dmsk);
+		int rc = dccp_feat_init(sk);
 
 		if (rc)
 			return rc;