nvme-auth: have dhchap_auth_work wait for queues auth to complete

It triggered the queue authentication work elements in parallel, but
the ctrl authentication work itself completes when all of them
completes. Hence wait for queues auth completions.

This also makes nvme_auth_stop simply a sync cancel of ctrl
dhchap_auth_work.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>