xfs: add a couple of queries to iterate free extents in the rtbitmap

Add _query_range and _query_all functions to the realtime bitmap
allocator.  These two functions are similar in usage to the btree
functions with the same name and will be used for getfsmap and scrub.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>