tpm: Lazily flush the auth session

Move the allocation of chip->auth to tpm2_start_auth_session() so that this
field can be used as flag to tell whether auth session is active or not.

Instead of flushing and reloading the auth session for every transaction
separately, keep the session open unless /dev/tpm0 is used.

Reported-by: Pengyu Ma <mapengyu@gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219229
Cc: stable@vger.kernel.org # v6.10+
Fixes: 7ca110f2

 ("tpm: Address !chip->auth in tpm_buf_append_hmac_session*()")
Tested-by: Pengyu Ma <mapengyu@gmail.com>
Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>