ima_tpm_chip is queried and saved only at IMA init, but never later

[Lakshmi Ramasubramanian <nramas@xxxxxxxxxxxxxxxxxxx>]

When IMA is initialized the default TPM is queried and saved.
If at this point a TPM wasn't available, but is detected and surfaced 
later by the system, IMA doesn't update the TPM information.

security/integrity/ima/ima_init.c
int __init ima_init(void)
{
	int rc;

	ima_tpm_chip = tpm_default_chip();
	if (!ima_tpm_chip)
		pr_info("No TPM chip found, activating TPM-bypass!\n");
	...
}

TPM PCR update function in IMA is essentially a NOP if TPM wasn't 
detected during IMA init.

security/integrity/ima/ima_queue.c
static int ima_pcr_extend(const u8 *hash, int pcr)
{
	int result = 0;

	if (!ima_tpm_chip)
		return result;
	...
}

In one configuration I am testing, I see the TPM appear post IMA Init. 
Likely this is rare, but I was wondering if there was a reason why TPM 
information is only queried during IMA init, but never updated at a 
later point.

Thanks,
 -lakshmi