dccp ccid-2: Separate option parsing from CCID processing

This patch replaces an almost identical replication of code: large parts
of dccp_parse_options() re-appeared as ccid2_ackvector() in ccid2.c.

Apart from the duplication, this caused two more problems:
 1. CCIDs should not need to be concerned with parsing header options;
 2. one can not assume that Ack Vectors appear as a contiguous area within an
    skb, it is legal to insert other options and/or padding in between. The
    current code would throw an error and stop reading in such a case.

Since Ack Vectors provide CCID-specific information, they are now processed
by the CCID directly, separating this functionality from the main DCCP code.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

net/dccp/ccids/ccid2.h

@@ -55,6 +55,7 @@ struct ccid2_seq {
  * @tx_rtt_seq:		     to decay RTTVAR at most once per flight
  * @tx_rpseq:		     last consecutive seqno
  * @tx_rpdupack:	     dupacks since rpseq
+ * @tx_av_chunks:	     list of Ack Vectors received on current skb
  */
 struct ccid2_hc_tx_sock {
 	u32			tx_cwnd;
@@ -79,6 +80,7 @@ struct ccid2_hc_tx_sock {
 	int			tx_rpdupack;
 	u32			tx_last_cong;
 	u64			tx_high_ack;
+	struct list_head	tx_av_chunks;
 };
 
 static inline bool ccid2_cwnd_network_limited(struct ccid2_hc_tx_sock *hc)