ANDROID: incremental fs: Fix race between truncate and write last block

Also fix race whereby multiple providers writinig the same block would
actually write out the same block.

Note that multiple_providers_test started failing when incfs was ported
to 5.15, and these fixes are needed to make the test reliable

Bug: 264703896
Test: incfs-test passes, specifically multiple_providers_test. Ran 100
      times
Change-Id: I05ad5b2b2f62cf218256222cecb79bbe9953bd97
Signed-off-by: Paul Lawrence <paullawrence@google.com>