ima: Use ima_show_htable_value to show violations and hash table data

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

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ima_show_htable_violations() and ima_show_measurements_count() both call
ima_show_htable_value() to copy the value of an atomic_long_t variable to
a buffer.

This patch modifies the definition of ima_show_htable_value(), so that this
function can be used in any file_operations structure. The atomic_long_t
variable used as source is chosen depending on the opened file in the
securityfs filesystem.

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>