mm/memory_hotplug.c: move register_memory_resource out of the lock_memory_hotplug
We don't need to do register_memory_resource() under
lock_memory_hotplug() since it has its own lock and doesn't make any
callbacks.
Also register_memory_resource return NULL on failure so we don't have
anything to cleanup at this point.
The reason for this rfc is I was doing some experiments with hotplugging
of memory on some of our larger systems. While it seems to work, it can
be quite slow. With some preliminary digging I found that
lock_memory_hotplug is clearly ripe for breakup.
It could be broken up per nid or something but it also covers the
online_page_callback. The online_page_callback shouldn't be very hard
to break out.
Also there is the issue of various structures(wmarks come to mind) that
are only updated under the lock_memory_hotplug that would need to be
dealt with.