Btrfs: fix clone ioctl where range is adjacent to extent

We had an edge case issue where the requested range was just
following an existing extent. Instead of skipping to the next
extent, we used the previous one which lead to having zero
sized extents.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>