ima: Allow choice of file hash algorithm for measurement and audit

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

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IMA reads the hash algorithm from security.ima, if exists, so that a
signature can be verified with the correct file digest.

This patch moves ima_read_xattr() and ima_get_hash_algo() to ima_main.c, so
that the file digest in the measurement list or in the audit logs can be
compared with a reference value calculated with a specific hash algorithm.

In addition, this patch also allows the usage of security.ima with type
EVM_IMA_XATTR_DIGSIG and signature length zero, so that the xattr can be
used just to specify the hash algorithm.

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>