memory-hotplug: use dev_online for memhp_auto_online

Commit 31bc3858ea ("add automatic onlining policy for the newly added
memory") provides the capability to have added memory automatically
onlined during add, but this appears to be slightly broken.

The current implementation uses walk_memory_range() to call
online_memory_block, which uses memory_block_change_state() to online
the memory.  Instead, we should be calling device_online() for the
memory block in online_memory_block().  This would online the memory
(the memory bus online routine memory_subsys_online() called from
device_online calls memory_block_change_state()) and properly update the
device struct offline flag.

As a result of the current implementation, attempting to remove a memory
block after adding it using auto online fails.  This is because doing a
remove, for instance

  echo offline > /sys/devices/system/memory/memoryXXX/state

uses device_offline() which checks the dev->offline flag.

Link: http://lkml.kernel.org/r/20170222220744.8119.19687.stgit@ltcalpine2-lp14.aus.stglabs.ibm.com
Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/base/memory.c

@@ -249,7 +249,7 @@ memory_block_action(unsigned long phys_index, unsigned long action, int online_t
 	return ret;
 }
 
-int memory_block_change_state(struct memory_block *mem,
+static int memory_block_change_state(struct memory_block *mem,
 		unsigned long to_state, unsigned long from_state_req)
 {
 	int ret = 0;

include/linux/memory.h

@@ -109,9 +109,6 @@ extern void unregister_memory_notifier(struct notifier_block *nb);
 extern int register_memory_isolate_notifier(struct notifier_block *nb);
 extern void unregister_memory_isolate_notifier(struct notifier_block *nb);
 extern int register_new_memory(int, struct mem_section *);
-extern int memory_block_change_state(struct memory_block *mem,
-				     unsigned long to_state,
-				     unsigned long from_state_req);
 #ifdef CONFIG_MEMORY_HOTREMOVE
 extern int unregister_memory_section(struct mem_section *);
 #endif

mm/memory_hotplug.c

@@ -1337,7 +1337,7 @@ int zone_for_memory(int nid, u64 start, u64 size, int zone_default,
 
 static int online_memory_block(struct memory_block *mem, void *arg)
 {
-	return memory_block_change_state(mem, MEM_ONLINE, MEM_OFFLINE);
+	return device_online(&mem->dev);
 }
 
 /* we are OK calling __meminit stuff here - we have CONFIG_MEMORY_HOTPLUG */