static int rxrpc_begin_rtt_probe(struct rxrpc_call *call, rxrpc_serial_t serial,
				 enum rxrpc_rtt_tx_trace why)
static void rxrpc_begin_rtt_probe(struct rxrpc_call *call, rxrpc_serial_t serial,
				  ktime_t now, enum rxrpc_rtt_tx_trace why)