SUNRPC: remove printk when back channel request not found

If the client interface is down, or there is a network partition between
the client and server that prevents the callback request to reach the
client, TCP on the server will keep re-transmitting the callback for about
~9 minutes before giving up and closing the connection.

If the connection between the client and the server is re-established
before the connection is closed and after the callback timed out (9 secs)
then the re-transmitted callback request will arrive at the client. When
the server receives the reply of the callback, receive_cb_reply prints the
"Got unrecognized reply..." message in the system log since the callback
request was already removed from the server xprt's recv_queue.

Even though this scenario has no effect on the server operation, a
malfunctioning or malicious client can fill up the server's system log.

Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>