Revert "sync: don't block the flusher thread waiting on IO"

This reverts commit 87a81e89c95a5616a4b80ebcd605e4c43c2449e5.
Revert "ext4: Speedup WB_SYNC_ALL pass called from sync(2)"

This reverts commit 438c3db5f93bdb841147b2aacd1cfc2c10598dc3.