ima: Fix build warnings

Fix build warnings (function parameters description) for
ima_collect_modsig(), ima_match_policy() and ima_parse_add_rule().

Fixes: 15588227 ("ima: Collect modsig") # v5.4+
Fixes: 2fe5d6de ("ima: integrity appraisal extension") # v5.14+
Fixes: 4af4662f ("integrity: IMA policy") # v3.2+
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>