nvme: expose TLS mode

nvme: expose TLS mode

It is not possible to determine the active TLS mode from the
presence or absence of sysfs attributes like tls_key,
tls_configured_key, or dhchap_secret.

With the introduction of the concat mode and optional DH-CHAP
authentication, different configurations can result in identical
sysfs state. This makes user space detection unreliable.

Expose the TLS mode explicitly to allow user space to
unambiguously identify the active configuration and avoid
fragile heuristics in nvme-cli.

Reviewed-by: Chris Leech <cleech@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Daniel Wagner <wagi@kernel.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>