rxrpc: Add tracepoint for ACK proposal

Add a tracepoint to log proposed ACKs, including whether the proposal is
used to update a pending ACK or is discarded in favour of an easlier,
higher priority ACK.

Whilst we're at it, get rid of the rxrpc_acks() function and access the
name array directly.  We do, however, need to validate the ACK reason
number given to trace_rxrpc_rx_ack() to make sure we don't overrun the
array.

Signed-off-by: David Howells <dhowells@redhat.com>

include/rxrpc/packet.h

@@ -123,6 +123,7 @@ struct rxrpc_ackpacket {
 #define RXRPC_ACK_PING_RESPONSE		7	/* response to RXRPC_ACK_PING */
 #define RXRPC_ACK_DELAY			8	/* nothing happened since received packet */
 #define RXRPC_ACK_IDLE			9	/* ACK due to fully received ACK window */
+#define RXRPC_ACK__INVALID		10	/* Representation of invalid ACK reason */
 
 	uint8_t		nAcks;		/* number of ACKs */
 #define RXRPC_MAXACKS	255