xfs: add a realtime flag to the bmap update log redo items

Extend the bmap update (BUI) log items with a new realtime flag that
indicates that the updates apply against a realtime file's data fork.
We'll wire up the actual code later.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>