kfence: make test case compatible with run time set sample interval

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4V388


CVE: NA

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The parameter kfence_sample_interval can be set via boot parameter and
late shell command, which is convenient for automated tests and KFENCE
parameter optimization.  However, KFENCE test case just uses
compile-time CONFIG_KFENCE_SAMPLE_INTERVAL, which will make KFENCE test
case not run as users desired.  Export kfence_sample_interval, so that
KFENCE test case can use run-time-set sample interval.

Signed-off-by: Peng Liu <liupeng256@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>