rxrpc: Replace call->acks_first_seq with tracking of the hard ACK point

rxrpc: Replace call->acks_first_seq with tracking of the hard ACK point

Replace the call->acks_first_seq variable (which holds ack.firstPacket from
the latest ACK packet and indicates the sequence number of the first ack
slot in the SACK table) with call->acks_hard_ack which will hold the
highest sequence hard ACK'd.  This is 1 less than call->acks_first_seq, but
it fits in the same schema as the other tracking variables which hold the
sequence of a packet, not one past it.

This will fix the rxrpc_congest tracepoint's calculation of SACK window
size which shows one fewer than it should - and will occasionally go to -1.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20241204074710.990092-21-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>