repair: switch block usage bitmap to a btree

repair: switch block usage bitmap to a btree

Using a btree representing the extents is much more space efficient than
using a bitmap tracking every single block.  In addition it also allows
for more optimal algorithms checking range overlaps instead of walking
every block in various places.

Also move the RT tracking bitmap into incore.c instead of leaving it
a as macros - this keeps the implementation contained.

Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>