mem-hotplug: introduce {un}lock_memory_hotplug()

mem-hotplug: introduce {un}lock_memory_hotplug()

Presently hwpoison is using lock_system_sleep() to prevent a race with
memory hotplug.  However lock_system_sleep() is a no-op if
CONFIG_HIBERNATION=n.  Therefore we need a new lock.

Signed-off-by: KOSAKI Motohiro <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Kamezawa Hiroyuki <[email protected]>
Suggested-by: Hugh Dickins <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>