folder_block_ops: reset `doDeferWrite` after recoverable error
folder_block_ops: reset `doDeferWrite` after recoverable error

This was leading to test flakes by leaving around dirty blocks in the
test cache, even after the test did its final sync.

Issue: KBFS-2093

keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs