nvme-auth: use chap->s2 to indicate bidirectional authentication

Commit 546dea18 ("nvme-auth: check chap ctrl_key once constructed")
replaced the condition "if (ctrl->ctrl_key)" (indicating bidirectional
auth) by "if (chap->ctrl_key)", because ctrl->ctrl_key is a resource shared
with sysfs. But chap->ctrl_key is set in
nvme_auth_process_dhchap_challenge() depending on the DHVLEN in the
DH-HMAC-CHAP Challenge message received from the controller, and will thus
be non-NULL for every DH-HMAC-CHAP exchange, even if unidirectional auth
was requested. This will lead to a protocol violation by sending a Success2
message in the unidirectional case (per NVMe base spec 2.0, the
authentication transaction ends after the Success1 message for
unidirectional auth). Use chap->s2 instead, which is non-zero if and only
if the host requested bi-directional authentication from the controller.

Fixes: 546dea18 ("nvme-auth: check chap ctrl_key once constructed")
Signed-off-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>