nvmet-auth: clean up with done_kfree

Jump directly to done_kfree to release d, which is consistent with the
code style behind.

Reported-by: Genjian Zhang <zhanggenjian@kylinos.cn>
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>