rcu: Consolidate debugging Kconfig options

The RCU-related debugging Kconfig options are in two different places,
and consume too much screen real estate.  This commit therefore
consolidates them into their own menu.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>