Don't fall back on 'cp' if the FICLONE ioctl gives an errno that's not
plausibly reflink specific, because in such a case any fallback could
theoretically mask real but intermittent system/storage errors.
Looking through ioctl_ficlone(2) and the kernel source, it should be
sufficient to do the fallback only on EBADF/EINVAL/EOPNOTSUPP/EXDEV.
(EISDIR/ETXTBSY don't apply to this storage driver, which will never
legitimately attempt to reflink a directory or an active - in the
storage domain - swap file.)
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