mm: memory_hotplug: wait on device_hotplug_lock in OOM path

Use lock_device_hotplug() inplace of lock_device_hotplug_sysfs() as the
latter can return immediately if there are any parallel
onlining/offlining of memory blocks.

Change-Id: Ica3cc286b7d4f87310f86e8c9e6db2157e511419
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>