nvmet-auth: authenticate on admin queue only

Do not start authentication on I/O queues as it doesn't really add value,
and secure concatenation disallows it anyway.  Authentication commands on
I/O queues are not aborted, so the host may still run the authentication
protocol on I/O queues.

Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>