[DCCP]: Miscellaneous code tidy-ups

This patch does not change code; it performs some trivial clean/tidy-ups:

  * removal of a `debug_prefix' string in favour of the
    already existing dccp_role(sk)

  * add documentation of structures and constants

  * separated out the cases for invalid packets (step 1
    of the packet validation)

  * removing duplicate statements

  * combining declaration & initialisation

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

net/dccp/output.c

@@ -333,6 +333,7 @@ struct sk_buff *dccp_make_response(struct sock *sk, struct dst_entry *dst,
 		return NULL;
 	}
 
+	/* Build and checksum header */
 	dh = dccp_zeroed_hdr(skb, dccp_header_size);
 
 	dh->dccph_sport	= inet_sk(sk)->sport;