xfs: check for shared rt extents when rebuilding rt file's data fork

xfs: check for shared rt extents when rebuilding rt file's data fork

When we're rebuilding the data fork of a realtime file, we need to
cross-reference each mapping with the rt refcount btree to ensure that
the reflink flag is set if there are any shared extents found.

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