checkpoint_gc: Exit gc if there is enough free segments

This script is primarily ensuring that there is sufficient storage
during the next boot while userdata is mounted with checkpoint=disable

If there are sufficient free segments, it won't matter if we've lost a
large number of blocks until boot complete, so we can exit early

Bug: 291181976
Test: Run checkpoint_gc with high fragmentation/low disk usage
Change-Id: Ia9725aafbd19ff576b9b6279a8e342a125d32fcc
1 file changed