releasetools: Fix the computation in ReviseStashSize().

We compute the max stashed_blocks in ReviseStashSize(), prior to calling
WriteTransfers(), to avoid running out of space due to stashing.

There is a bug when computing the to-be-freed stashed blocks, where we
wrongly free the space _before_ executing the transfer command. This leads
to a script failure where the max stash size violates the max allowed
size in WriteTransfers().

Note that this bug doesn't affect already generated packages. It's only
an underestimate in ReviseStashSize(). The check in WriteTransfers() has
been correct to ensure the max stash size.

Bug: 33687949
Test: Successfully generated incremental OTA which failed previously.
Change-Id: I4f4f043c6f521fce81ca5286e6156f22d99bf7f7