kasan: Zero shadow memory on memory hotplug

Vmalloc does not zero memory by default. In this instance,
CONFIG_PAGE_POISONING was enabled, so the pages were initialized
to 0xAA.

 kasan_report+0x28c/0x2f8
 check_memory_region+0x1dc/0x1e8
 memset+0x2c/0x50
 kernel_poison_pages+0x234/0x398
 __free_pages_ok+0x380/0xd50
 __free_pages+0x4c/0xd0
 online_pages_range+0x174/0x1c8
 walk_system_ram_range+0x130/0x170
 online_pages+0x298/0x458
 memory_block_change_state+0x160/0x210
 memory_subsys_online+0x68/0x90
 device_online+0x9c/0xc8
 add_memory_resource+0x18c/0x228
 add_memory+0x118/0x180

Memory state around the buggy address:
 ffffffc17fffff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffffffc17fffff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffffffc180000000: aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
                   ^
 ffffffc180000080: aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
 ffffffc180000100: aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa

Change-Id: Id487a53d3cbe14a4ba73d43e151a51fd3162b1e0
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>