nvmet-auth: always free derived key data

After calling nvme_auth_derive_tls_psk() we need to free the resulting
psk data, as either TLS is disable (and we don't need the data anyway)
or the psk data is copied into the resulting key (and can be free, too).

Fixes: fa2e0f8b ("nvmet-tcp: support secure channel concatenation")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Suggested-by: Maurizio Lombardi <mlombard@bsdbackstore.eu>
Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>