rxrpc: Note serial number being ACK'd in the congestion management trace

Note the serial number of the packet being ACK'd in the congestion
management trace rather than the serial number of the ACK packet.  Whilst
the serial number of the ACK packet is useful for matching ACK packet in
the output of wireshark, the serial number that the ACK is in response to
is of more use in working out how different trace lines relate.

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