filesystem: preserve metadata file permissions on updates

Since fscrypt replaces metadata files rather than overwrites them (to
get atomicity), their owner will change to root if root makes a change.
That isn't too much of an issue when the files have mode 0644.  However,
it will become a much bigger issue when the files have mode 0600,
especially because existing files with mode 0644 would also get changed
to have mode 0600.

In preparation for this, start preserving the previous owner and mode of
policy and protector files when they are updated.

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