nvme-sysfs: add 'tls_keyring' attribute

Add a 'tls_keyring' attribute to display the contents of the
--keyring option from the connect string. Adding this attribute
allows us to recreate the original connect string from sysfs
settings.

Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>