xfs: teach rmapbt to support interval queries

Now that the generic btree code supports querying all records within a
range of keys, use that functionality to allow us to ask for all the
extents mapped to a range of physical blocks.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>