: Sync/Reset Floods; Rate-Limiting for DCCP-Sync

This patch set fixes two bugs and adds rate-limiting for DCCP-Syncs.

Patch #1: Uses variables instead of a reference to skb fields;
it is needed by patch #2 (no code change here).

Patch #2: Fixes the Sync/Reset flood bug discussed earlier on
dccp@vger, by adding the special case for sequence-invalid Reset from RFC 4340, 7.5.4.

Patch #3: Renames seq' parameter of dccp_send_sync into ackno'.

Patch #4: Implements rate-limiting for DCCP-Syncs.

Patch #5: Adds another exception required by RFC 4340, 7.5.4 -
In state REQUEST, receiving a Sync generates a Reset and not a SyncAck.