rxrpc: Fix return from none_validate_challenge()

rxrpc: Fix return from none_validate_challenge()

Fix the return value of none_validate_challenge() to be explicitly true
(which indicates the source packet should simply be discarded) rather than
implicitly true (because rxrpc_abort_conn() always returns -EPROTO which
gets converted to true).

Note that this change doesn't change the behaviour of the code (which is
correct by accident) and, in any case, we *shouldn't* get a CHALLENGE
packet to an rxnull connection (ie. no security).

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lists.infradead.org/pipermail/linux-afs/2025-April/009738.html
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/10720.1748358103@warthog.procyon.org.uk
Fixes: 5800b1cf3fd8 ("rxrpc: Allow CHALLENGEs to the passed to the app for a RESPONSE")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>