rcutorture: Refactor TREE_RCU test cases

The TREE_RCU test cases were first put in place many years ago, and have
been incrementally modified rather than being reworked.  This commit
therefore completes a long-overdue reworking of the TREE_RCU test cases.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>