imans: Use initial ima namespace domain tag when IMANS is disabled.

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4C9AE


CVE: NA

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As part of the imans support, a key domain tag is added to the search
criteria in digsig module. When IMA Namespace is disabled, the initial
ima namespace domain tag should be used instead of nsproxy.

Signed-off-by: Ajo Jose Panoor <ajo.jose.panoor@huawei.com>
Reviewed-by: Zhang Tianxing <zhangtianxing3@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>