When we descend the tree to find our slot, if we step to the right, we
are no longer the leftmost node.

Fixes: f808c13f ("lib/interval_tree: fast overlap detection")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Christian König <christian.koenig@amd.com> for now.
Link: https://patchwork.freedesktop.org/patch/msgid/20180220093738.1461-1-chris@chris-wilson.co.uk