ssd: attach LAB_NODE_SSD_ROOT to ssd->tree

ssd: attach LAB_NODE_SSD_ROOT to ssd->tree

This doesn't change any behaviors.

Attaching LAB_NODE_NONE node-descriptor to ssd->tree looks strange, this
patch uses new LAB_NODE_SSD_ROOT instead. The node-descriptor attached to
ssd->tree is needed for get_cursor_context() to detect cursor hovering on
borders/extents.

I also updated get_cursor_context() to make my intent clearer.