When file opened with O_TMPFILE but use_tmpfile==0, the file will not be
linked to the directory (the code at the end of process_one_file_reg).
Additionally it is waste of time trying using O_TMPFILE when it's
already known it shouldn't be.
Also use_tmpfile==0 can mean we don't have access to /proc
(set_procfs_fd wasn't called), so even if linking the file to its
directory would be attempted, it would fail. This is the case for
dom0-updates copy.
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