rcu: Add #ifdef to suppress __rcu_offline_cpu() warning in !HOTPLUG_CPU builds

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
Cc: Paul Mundt <lethal@linux-sh.org>
LKML-Reference: <20090825154025.GD6616@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>