nvmet-auth: assign dh_key to NULL after kfree_sensitive

ctrl->dh_key might be used across multiple calls to nvmet_setup_dhgroup()
for the same controller. So it's better to nullify it after release on
error path in order to avoid double free later in nvmet_destroy_auth().

Found by Linux Verification Center (linuxtesting.org) with Svace.

Fixes: 7a277c37 ("nvmet-auth: Diffie-Hellman key exchange support")
Cc: stable@vger.kernel.org
Signed-off-by: Vitaliy Shevtsov <v.shevtsov@maxima.ru>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>