mm, memory_hotplug: enable kernel mappings during online pages

Previously, the kernel mappings are enabled in the offline path, after the
pages are offlined and isolated, using post_alloc_hook. This could be time
consuming and is done under zone->lock taken which could cause stalls for
tasks which are spinning waiting for this lock. Also, having the kernel
mappings for offlined pages could cause unintended speculative accesses on
dead DDR region. Instead, enable the kernel mappings in the online path
before the pages are freed to buddy system.

Change-Id: Iccd542fe9e1a90a0a774f5cbe4cc4095dbaee185
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>