xfs: improve busy extent sorting

Sort busy extents by the full block number instead of just the AGNO so
that we can issue consecutive discard requests that the block layer could
merge (although we'll need additional block layer fixes for fast devices).

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