tpm: Return tpm2_sessions_init() when null key creation fails

Do not continue tpm2_sessions_init() further if the null key pair creation
fails.

Cc: stable@vger.kernel.org # v6.10+
Fixes: d2add27c ("tpm: Add NULL primary creation")
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>