Fix the service handler function for the CB.ProbeUuid RPC call so that it
replies in the correct manner - that is an empty reply for success and an
abort of 1 for failure.

Putting 0 or 1 in an integer in the body of the reply should result in the
fileserver throwing an RX_PROTOCOL_ERROR abort and discarding its record of
the client; older servers, however, don't necessarily check that all the
data got consumed, and so might incorrectly think that they got a positive
response and associate the client with the wrong host record.

If the client is incorrectly associated, this will result in callbacks
intended for a different client being delivered to this one and then, when
the other client connects and responds positively, all of the callback
promises meant for the client that issued the improper response will be
lost and it won't receive any further change notifications.

Fixes: 9396d496 ("afs: support the CB.ProbeUuid RPC op")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>