nvme-tcp: check for invalidated or revoked key

key_lookup() will always return a key, even if that key is revoked
or invalidated. So check for invalid keys before continuing.

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