xfs: reuse xfs_bmap_update_cancel_item

Reuse xfs_bmap_update_cancel_item to put the AG/RTG and free the item in
a few places that currently open code the logic.

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