There are no more (known) nested calls to get_online_cpus() and all
observed lock ordering problems have been addressed.

Replace the magic nested 'rwsem' hackery with a percpu-rwsem.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Tested-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NIngo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20170524081549.447014063@linutronix.de