ima: Add a list of the installed ima namespaces

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I49KW1


CVE: NA

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Add a list of the installed ima namespaces. IMA namespace is considered
installed, if there is at least one process born in that namespace.

This list will be used to check the read-write violations and to detect
any object related changes relevant across namespaces.

Signed-off-by: Krzysztof Struczynski <krzysztof.struczynski@huawei.com>
Reviewed-by: Zhang Tianxing <zhangtianxing3@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>