nvme-core: fix memory leak in dhchap_ctrl_secret

Free dhchap_secret in nvme_ctrl_dhchap_ctrl_secret_store() before we
return when nvme_auth_generate_key() returns error.

Fixes: f50fff73 ("nvme: implement In-Band authentication")
Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>