The previous code incorrectly rounded down the number of entries to free
to kBlockSize (expressed in KB) instead of kEntriesPerBlock (expressed
in # of entries) to compute the start of the evacuation area. Further,
depending on the block sized used, the previous heuristics does not
necessarily guarantee that at least one full block would be evacuated.
This CL fixes both of these issues.

Bug: v8:10391
Change-Id: I5ddecd5d582bcf89e1c52df431f006889685320a
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3837860Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82557}