initramfs: add file metadata

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

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This patch adds metadata to a file from a supplied buffer. The buffer might
contains multiple metadata records. The format of each record is:

<metadata len (ASCII, 8 chars)><version><type><metadata>

For now, only the TYPE_XATTR metadata type is supported. The specific
format of this metadata type is:

<xattr #N name>\0<xattr #N value>

[kamensky: fixed restoring of xattrs for symbolic links by using
           sys_lsetxattr() instead of sys_setxattr()]

[sassu: removed state management, kept only do_setxattrs(), added support
        for generic file metadata, replaced sys_lsetxattr() with
        vfs_setxattr(), added check for entry_size, added check for
        hdr->c_size, replaced strlen() with strnlen(); moved do_setxattrs()
        before do_name()]

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Victor Kamensky <kamensky@cisco.com>
Signed-off-by: Taras Kondratiuk <takondra@cisco.com>
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>