ima: Define and use a digest_size field in the ima_algo_desc structure

Add the digest_size field to the ima_algo_desc structure to determine the
digest size from the correct source.

If the hash algorithm is among allocated PCR banks, take the value from the
TPM bank info (equal to the value from the crypto subsystem if the TPM
algorithm is supported by it; otherwise, not exceding the size of the
digest buffer in the tpm_digest structure, used by IMA).

If the hash algorithm is SHA1, use the predefined value. Lastly, if the
hash algorithm is the default one but not among the PCR banks, take the
digest size from the crypto subsystem (the default hash algorithm is
checked when parsing the ima_hash= command line option).

Finally, use the new information to correctly show the template digest in
ima_measurements_show() and ima_ascii_measurements_show().

Link: https://github.com/linux-integrity/linux/issues/14
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>