user namespace: Add function that checks if the UID map is defined

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


CVE: NA

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Add function that checks if the UID map is defined. It will be used by
ima to check if ID remapping in subject-based rules is necessary.

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