is not monotonically increasing (e.g. clock is slaved to another system)
the optimization will result in segments being treated as corrupt
(uncleanable). If enough such "bad" segments were created, the cleaner would
clean continuously, and after some time the system would panic with "no
clean segments".
(Legitimately old partial-segments are relatively rare, and will have their
blocks culled by lfs_bmapv.)
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