rcu: Don't disable CPU hotplug during OOM notifiers

RCU's rcu_oom_notify() disables CPU hotplug in order to stabilize the
list of online CPUs, which it traverses.  However, this is completely
pointless because smp_call_function_single() will quietly fail if invoked
on an offline CPU.  Because the count of requests is incremented in the
rcu_oom_notify_cpu() function that is remotely invoked, everything works
nicely even in the face of concurrent CPU-hotplug operations.

Furthermore, in recent kernels, invoking get_online_cpus() from an OOM
notifier can result in deadlock.  This commit therefore removes the
call to get_online_cpus() and put_online_cpus() from rcu_oom_notify().

Reported-by: Marcin Ślusarz <marcin.slusarz@gmail.com>
Reported-by: David Rientjes <rientjes@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: David Rientjes <rientjes@google.com>
Tested-by: Marcin Ślusarz <marcin.slusarz@gmail.com>