ima: Introduce new securityfs files

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

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This patch introduces three new files in the securityfs filesystem.
digest_list_data: loads a digest list from the specified path and adds the
digests to the hash table; digest_list_data_del: does the same but removes
the digests from the hash table; digests_count: shows the current number of
digests stored in the hash table.

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>