nvmet-auth: fix nvmet_auth hash error handling

If we fail to call nvme_auth_augmented_challenge, or fail to kmalloc
for shash, we should free the memory allocation for challenge, so add
err path out_free_challenge to fix the memory leak.

Fixes: 7a277c37 ("nvmet-auth: Diffie-Hellman key exchange support")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>