[DCCP]: Fix Reset/Sync-Flood Bug

[DCCP]: Fix Reset/Sync-Flood Bug

This updates sequence number checking with regard to RFC 4340, 7.5.4.
Missing in the code was an exception for sequence-invalid Reset packets,
which get a Sync acknowledging GSR, instead of (as usual) P.seqno.

This can lead to an oscillating ping-pong flood of Reset packets.

In fact, it has been observed on the wire as follows:

 1. client establishes connection to server;
 2. before server can write to client, client crashes without notifying
    the server (NB: now no longer possible due to ABORT function);
 3. server sends DCCP-Data packet (has no ackno);
 4. client generates Reset "No Connection", seqno=0, increments seqno;
 5. server replies with Sync, using ackno = P.seqno;
 6. client generates Reset "No Connection" with seqno = ackno + 1;
 7. goto (5).

The difference is that now in (5) the server uses GSR.  This causes the
Reset sent by the client in (6) to become sequence-valid, so that in (7)
the vicious circle is broken; the Reset is then enqueued and causes the
socket to enter TIMEWAIT state.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: David S. Miller <davem@davemloft.net>