initramfs: read metadata from special file METADATA!!!

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

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Instead of changing the CPIO format, metadata are parsed from regular files
with special name 'METADATA!!!'. This file immediately follows the file
metadata are added to.

This patch checks if the file being extracted has the special name and, if
yes, creates a buffer with the content of that file and calls
do_parse_metadata() to parse metadata from the buffer.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Tianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>