ima: Generalize ima_write_policy() and raise uploaded data size limit

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

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ima_write_policy() is being used to load a new policy from user space. This
function can be reused to load different types of data.

This patch renames ima_write_policy() to ima_write_data() and executes the
appropriate actions depending on the opened file in securityfs.

Also, this patch raises the uploaded data size limit to 64M, to accept
files (e.g. digest lists) larger than a policy. The same limit is used for
the SELinux policy.

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>