Fix cow v3 size estimation errors

Current cow size estimation is computed by taking the offset of next
data write. However, during COW size estimation we repeatdly increment
op_count_max. Since data section is placed right after op section,
incrementing  op_count_max has the effect of moving the beginning of
data section. next_data_pos_ is never updated in this process, causing
the final estiamte to be off.

Test: th
Bug: 313962438
Change-Id: I250dff54c470c9c20d6db33d91bac898358dee31