nvme-keyring: register '.nvme' keyring

Register a '.nvme' keyring to hold keys for TLS and DH-HMAC-CHAP and
add a new config option NVME_KEYRING.
We need a separate keyring for NVMe as the configuration is done
via individual commands (eg for configfs), and the usual per-session
or per-process keyrings can't be used.

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