[DCCP] CCID2: Allocate seq records on demand

Allocate more sequence state on demand.  Each time a packet is sent
out by CCID2, a record of it needs to be kept.  This list of records
grows proportionally to cwnd.  Previously, the length of this list was
hardcored and therefore the cwnd could only grow to this value (of
128).  Now, records are allocated on demand as necessary---cwnd may
grow as it wishes.  The exceptional case of when memory is not
available is not handled gracefully.  Perhaps, cwnd should be capped
at that point.

Signed-off-by: Andrea Bittau <a.bittau@cs.ucl.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>