ima: Add parser of compact digest list

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

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This patch introduces the parser of the compact digest list. The format is
optimized to store a large quantity of data with the same type. It is the
only format supported by the kernel. Digest lists can be uploaded by
writing the path to securityfs, as the same as for IMA policies.

A compact list is a set of consecutive data blocks, each consisting of a
header and a payload. The header indicates the version of the header, the
type of data, type modifiers, the hash algorithm, how many elements and the
length of the payload.

COMPACT_KEY identifies public keys used for signature verification of the
digest lists; COMPACT_PARSER identifies digests of user space parsers
allowed to directly upload parsed digest lists to the kernel; COMPACT_FILE
identifies digests of regular files; COMPACT_METADATA identifies digest of
file metadata.

Type modifiers indicate attributes of the elements included in the payload.
The COMPACT_MOD_IMMUTABLE modifier indicates that a file or metadata are
immutable.

This patch also introduces ima_lookup_loaded_digest() and
ima_add_digest_data_entry() to search and add digests in the new hash table
(ima_digests_htable).

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>