rxrpc: Fix response to PING RESPONSE ACKs to a dead call

Stop rxrpc from sending a DUP ACK in response to a PING RESPONSE ACK on a
dead call.  We may have initiated the ping but the call may have beaten the
response to completion.

Fixes: 18bfeba5

 ("rxrpc: Perform terminal call ACK/ABORT retransmission from conn processor")
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
Signed-off-by: David S. Miller <davem@davemloft.net>