rxrpc: Fix error when reading rxrpc tokens

When converting from ASSERTCMP to WARN_ON, the tested condition must
be inverted, which was missed for this case.

This would cause an EIO error when trying to read an rxrpc token, for
instance when trying to display tokens with AuriStor's "tokens" command.

Fixes: 84924aac ("rxrpc: Fix checker warning")
Signed-off-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>