drivers/base/memory.c: don't access uninitialized memmaps in soft_offline_page_store()

Uninitialized memmaps contain garbage and in the worst case trigger kernel
BUGs, especially with CONFIG_PAGE_POISONING. They should not get touched.

Right now, when trying to soft-offline a PFN that resides on a memory
block that was never onlined, one gets a misleading error with
CONFIG_PAGE_POISONING:

:/# echo 5637144576 > /sys/devices/system/memory/soft_offline_page
[ 23.097167] soft offline: 0x150000 page already poisoned

But the actual result depends on the garbage in the memmap.

soft_offline_page() can only work with online pages, it returns -EIO in
case of ZONE_DEVICE. Make sure to only forward pages that are online
(iow, managed by the buddy) and, therefore, have an initialized memmap.

Add a check against pfn_to_online_page() and similarly return -EIO.

Link: http://lkml.kernel.org/r/[e-mail]
Fixes: f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded memory to zones until online") [visible after d0dc12e86b319]
Signed-off-by: David Hildenbrand <[e-mail]>
Acked-by: Naoya Horiguchi <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: <[e-mail]> [4.13+]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/base/memory.c

···
 	pfn >>= PAGE_SHIFT;
 	if (!pfn_valid(pfn))
 		return -ENXIO;
+	/* Only online pages can be soft-offlined (esp., not ZONE_DEVICE). */
+	if (!pfn_to_online_page(pfn))
+		return -EIO;
 	ret = soft_offline_page(pfn_to_page(pfn), 0);
 	return ret == 0 ? count : ret;
 }