Changes for 4.19:

- Use extent maps to track pagecache page status instead of bufferhead
  state.
- Refactor pagecache read and write paths to use the new iomap library
  functions, which enable us to drop the old bufferhead code for
  pagesize == blocksize filesystems.
- Set up parallel per-block-per-page metadata to track subpage
  information that was tracked by buffer heads, which enables us to drop
  the old bufferhead code for pagesize > blocksize filesystems.
- Tie a deferred ops control structure to a transaction so that we can
  take advantage of an upper-level dfops without having to plumb pointer
  passing through the code.
- Refactor the deferred ops code to track deferred ops as part of the
  transaction structure (instead of as a separate data structure) so
  that we can simplify the scoping rules around defer_ops.
- Refactor twisty delwri buffer submission code to avoid deadlocks.
- Shorten and fix indenting problems in the scrub code.
- Detect obviously bad summary counts at mount and fix them.
- Directly associate deferred ops control structure with a transaction
  so that callers no longer have to manage it themselves.
- Remove a couple of IRIX-era inode macros.
- Remove the long-deprecated 'barrier' and 'nobarrier' mount options.
- Clean up the inode fork structure a bit.
- Check for bad fs summary counter values in the superblock.
- Reduce COW fork lookups during writeback.
- Refactor the deferred ops control structures into the transaction
  structure, thereby eliminating the need for transaction users to
  handle the deferred ops as a separate data structure.
- Add the ability to repair AG headers online.
- Fix a crash due to insufficient return value checking.
- Various fixes and cleanups.
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