xfs: add shared rmap map/unmap/convert log item types

Wire up some rmap log redo item type codes to map, unmap, or convert
shared data block extents.  The actual log item recovery comes in a
later patch.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>