rx: Ignore responses to nonexistent challenges

rx: Ignore responses to nonexistent challenges

Consider the following situation:

 - A client sends a data packet to a server, using a security class
   that requires a challenge
 - The server responds with a challenge
 - The server is restarted
 - The client responds to the challenge with a response

In that situation, the server will process the response, but since the
server was restarted, it has no knowledge of the challenge that was
sent. This generally means that we error the connection, since the
given response is not valid. For rxkad with modern endpoints, this
results in an RXKADPACKETSHORT error, since we interpret the response
as an 'old' response, but it's actually a 'v2' response, so we
interpret the fields in the response as garbage.

This means that the client gets a connection error when the client did
nothing wrong, and there's no way for the client to distinguish this
from a real connection error.

One way to solve this would be to send a Challenge packet to the
client immediately when we detect that this situation has occurred.
However, if we do that, then we never see a data packet with a
checksum, so we fall back to using "old" challenges and responses. And
in general, that would cause the server side to never see a data
packet during the connection negotiation, which is unusual and I am
concerned there may be other niggles of odd behavior that may occur in
that scenario.

So instead, to fix this, make the server ignore responses in this
situation (that is, if we haven't sent out any challenges yet).
Clients will eventually resend the data packet, and we will go through
negotiating the connection security like normal. This should never
cause any new problems, since dropping a challenge packet must be
handled anyway (sometimes packets just get dropped). And a client will
never hang on sending the same response over and over again; clients
only ever send a Response in response to a Challenge packet.

Change-Id: Id3fae425addb2ac8ab60965213b3ebca2e64ba5d
Reviewed-on: http://gerrit.openafs.org/10315
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>