(More) new code for 5.7:

- Validate the realtime geometry in the superblock when mounting
- Refactor a bunch of tricky flag handling in the log code
- Flush the CIL more judiciously so that we don't wait until there are
  millions of log items consuming a lot of memory.
- Throttle transaction commits to prevent the xfs frontend from flooding
  the CIL with too many log items.
- Account metadata buffers correctly for memory reclaim.
- Mark slabs properly for memory reclaim.  These should help reclaim run
  more effectively when XFS is using a lot of memory.
- Don't write a garbage log record at unmount time if we're trying to
  trigger summary counter recalculation at next mount.
- Don't block the AIL on locked dquot/inode buffers; instead trigger its
  backoff mechanism to give the lock holder a chance to finish up.
- Ratelimit writeback flushing when buffered writes encounter ENOSPC.
- Other minor cleanups.
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