ima: fix violation measurement list record

Although the violation digest in the IMA measurement list is always
zeroes, the size of the digest should be based on the hash algorithm.
Until recently the hash algorithm was hard coded to sha1.  Fix the
violation digest size included in the IMA measurement list.

This is just a cosmetic change which should not affect attestation.

Reported-by: Stefan Berger <stefanb@linux.ibm.com>
Fixes: 09091c44 ("ima: use IMA default hash algorithm for integrity violations")
Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>