mkfs.f2fs: discard obsolete blocks to avoid roll-forward recovery
mkfs.f2fs: discard obsolete blocks to avoid roll-forward recovery

If there are many obsolete blocks written by previous usage, we can recover
wrong blocks with them.
So, mkfs should discard whole wrong information correctly by removing possible
same checkpoint versions.

Signed-off-by: Jaegeuk Kim <71255387a32c360dfbe6dfb7aaeb123bf1dc8cb6@kernel.org>

TurboROM/external_f2fs-tools,seamlik/f2fs-tools,seamlik/f2fs-tools,TurboROM/external_f2fs-tools