CVE-2026-74430: rxrpc: Fix ACKALL packet handling

In the Linux kernel, the following vulnerability has been resolved:

rxrpc: Fix ACKALL packet handling

rxrpcinputackall() accepts ACKALL packets without checking whether the call is in a state that can legitimately have outstanding transmit buffers. A forged ACKALL can therefore reach a new service call in RXRPCCALLSERVERRECVREQUEST before any reply packets have been queued.

In that state call->txtop is zero and call->txqueue is NULL, so rxrpcrotatetxwindow() dereferences a NULL txqueue and triggers a null-pointer dereference.

Fix the handling of ACKALL packets by the following means:

(1) Add two new call states: RXRPCCALLCLIENTPRESEND which indicates that the client call is connected, but nothing has been transmitted as yet; and RXRPCCALLCLIENTAWAITACK, which indicates that everything has been transmitted at least once, but we're now waiting for the stuff remaining in the Tx buffer to be ACK'd (retransmissions may still happen).

The RXRPCCALLCLIENTPRESEND state is set when the call is assigned a channel and transitions to RXRPCCALLCLIENTSENDREQUEST when the first packet is transmitted.

RXRPCCALLCLIENTAWAITREPLY is then narrowed in scope to indicate that all Tx packets have been ACK'd and we're now waiting for the reply to be received.

(2) As per Wyatt Feng's original patch[1], the ACKALL handler then checks that the call state is one in which there might be stuff in the Tx buffer to ACK, but now this includes AWAITACK rather than AWAITREPLY. ACKALL packets are ignored if received in the wrong state.

Note that unlike Wyatt Feng's patch, it's no longer necessary to check to see if the Tx buffer exists as this the state set now covers this.

(3) Make the ACKALL handler use call->txtransmitted rather than call->txtop as the former is explicitly the highest packet seq number transmitted, whereas the latter has a looser definition.

Thanks to Jeffrey Altman for a description of the history of the ACKALL packet[1].