rcu: move Kconfig menu

Move RCU Kconfig options from top-level menu to an "RCU Subsystem"
menu under the "General Setup" menu.

Signed-off-by: Mike Travis <travis@sgi.com>
Tested-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>