KEYS: trusted: Fix tpm2_load_cmd() boundary check

tpm2_load_cmd() does boundary checks against the ASN.1 size i.e.,
payload->blob_len. Address this by passing the decoded blob size to
tpm2_load_cmd(), and use it for the boundary checks.

Cc: stable@vger.kernel.org # v5.13+
Fixes: f2219745250f ("security: keys: trusted: use ASN.1 TPM2 key format for the blobs")
Reported-by: co+6a581c4284f721d4@bugs.sh
Closes: https://bugs.sh/b/6a581c4284f721d4/
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Srish Srinivasan <ssrish@linux.ibm.com>
Link: https://lore.kernel.org/r/20260901205809.2028454-1-jarkko@kernel.org
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>