Plumb in the pieces (init_high_key, diff_two_keys) necessary to call
query_range on the free space btrees.  Remove the debugging asserts
so that we can make queries starting from block 0.

While we're at it, merge the redundant "if (btnum ==" hunks.

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