xfs: widen BUI formats to support realtime

Atomic extent swapping (and later, reverse mapping and reflink) on the
realtime device needs to be able to defer file mapping and extent
freeing work in much the same manner as is required on the data volume.
Make the BUI log items operate on rt extents in preparation for atomic
swapping and realtime rmap.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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