ima: Add support for appraisal with digest lists

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

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IMA-Appraise grants access to files with a valid signature or with actual
file digest equal to the digest included in security.ima.

This patch adds support for appraisal based on digest lists. Instead of
using the reference value from security.ima, this patch checks if the
calculated file digest is included in the uploaded digest lists.

This functionality must be explicitly enabled by providing one of the
following values for the ima_appraise_digest_list= kernel option:

- digest: this mode enables appraisal verification with digest lists until
  EVM is initialized; after that, EVM verification must be successful even
  if the file digest is found in a digest list;

- digest-nometadata: this mode enables appraisal verification with digest
  lists even after EVM has been initialized; files without security.evm are
  allowed if the digest of the content is found in the digest list, and
  security.evm is created with current values of xattrs (trust at first
  use); all files created in this way will have the new security.ima type
  EVM_IMA_XATTR_DIGEST_LIST; they can be accessed later only if this mode
  has been selected.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Tianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>