rxrpc: Don't use received skbuff timestamps

Don't use received skbuff timestamps, but rather set a timestamp when an
ack is processed so that the time taken to get to rxrpc_input_ack() is
included in the RTT.

The timestamp of the latest ACK received is tracked in
call->acks_latest_ts.

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/20241204074710.990092-26-dhowells@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>