xfs: replace xfs_qm_get_rtblks with a direct call to xfs_bmap_count_leaves

Use the existing functionality instead of directly poking into the extent
list.

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>