ima: Generalize ima_read_policy()

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

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This patch renames ima_read_policy() to ima_read_file() so that the
function can be used to read files for different purposes. It also adds the
opened file in securityfs as parameter so that the function can determine
which action it should do with the passed data.

This patch replaces kernel_read_file_from_path() with filp_open() +
kernel_read_file() so that the file descriptor can be used for further
checks.

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>