Changes since last update:
- Fix a locking problem during xattr block conversion that could lead to
  the log checkpointing thread to try to write an incomplete buffer to
  disk, which leads to a corruption shutdown
- Fix a null pointer dereference when removing delayed allocation extents
- Remove post-eof speculative allocations when reflinking a block past
  current inode size so that we don't just leave them there and assert on
  inode reclaim
- Relax an assert which didn't accurately reflect the way locking works
- Avoid infinite loop when cancelling copy on write extents after a
  writeback failure
- Try to avoid copy on write transaction reservation overflows when
  remapping after a successful write