rcu: Split hierarchical RCU initialization into boot-time and CPU-online pieces

This patch divides the rcutree initialization into boot-time
and hotplug-time components, so that the tree data structures
are guaranteed to be fully linked at boot time regardless of
what might happen in CPU hotplug operations.

This makes RCU more resilient against CPU hotplug misbehavior
(and vice versa), but more importantly, does a better job of
compartmentalizing the code.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: josht@linux.vnet.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: hugh.dickins@tiscali.co.uk
Cc: benh@kernel.crashing.org
LKML-Reference: <1250355231152-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>