ima: Generalize policy file operations

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

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This patch renames ima_open_policy() and ima_release_policy() respectively
to ima_open_data_upload() and ima_release_data_upload(). They will be used
to implement file operations for interfaces allowing to load data from user
space.

A new flag (IMA_POLICY_BUSY) has been defined to prevent concurrent policy
upload.

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>