repair: optimize duplicate extent tracking

repair: optimize duplicate extent tracking

Switch the duplicate extent tracking from an avl tree to our new btree
implementation.  Modify search_dup_extent to find overlapping extents
with differening start blocks instead of having the caller walk every
possible start block.

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