rcu: Move RCU non-debug Kconfig options to kernel/rcu

RCU's Kconfig options are scattered, and there are enough of them
that it would be good for them to be more centralized.  This commit
therefore extracts RCU's Kconfig options from init/Kconfig into a new
kernel/rcu/Kconfig file.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>