rcu: Add CPU-offline processing for single-node configurations

Add preemptable-RCU plugin to handle the CPU-offline
processing.

An additional plugin is forthcoming to handle multinode RCU
trees, but this current plugin works for configurations up to
32 CPUs (64 CPUs for 64-bit kernels).

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josht@linux.vnet.ibm.com
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
LKML-Reference: <12511321213336-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>