rxrpc: Fix the names of the fields in the ACK trailer struct

From AFS-3.3 a trailer containing extra info was added to the ACK packet
format - but AF_RXRPC has the names of some of the fields mixed up compared
to other AFS implementations.

Rename the struct and the fields to make them match.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org