- Reports realtime device free blocks in statfs calls if (realtime)
  inheritance bit is set on the inode of directory, or realtime flag
  in the case of files.  This is a bit more intuitive, especially for
  use-cases which are using a much larger device for the realtime device.
- Add XFS_IS_REALTIME_MOUNT option to gate based on the existence of a
  realtime device on the mount, similar to the XFS_IS_REALTIME_INODE
  option.
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDave Chinner <dchinner@redhat.com>
Signed-off-by: NRichard Wareing <rwareing@fb.com>
Reviewed-by: NDarrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>