evm: Add support for digest lists of metadata

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

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This patch adds support in EVM to verify file metadata digest with digest
lists. Metadata digest, calculated in the same way as for portable
signatures, is searched in the digest lists only if the file has the
security.evm xattr with type EVM_IMA_XATTR_DIGEST_LIST.

If the found digest is marked as immutable, content and xattr/attr updates
are not allowed. Otherwise, after verification, the existing security.evm
with the new type will be replaced with an HMAC, similarly to non-portable
signatures.

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>