Revert "mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug_lock"

Revert "mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug_lock"

BugLink: https://bugs.launchpad.net/bugs/1862312
This reverts commit 483735440add2530c03da656ede83ee81414a566.

The backport for upstream commit 381eab4a6ee8 (mm/memory_hotplug: fix
online/offline_pages called w.o. mem_hotplug_lock) placed the call to
mem_hotplug_begin() on the wrong location and caused a regression.

Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Sultan Alsawaf <sultan.alsawaf@canonical.com>
Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>