mm/memory_hotplug: fix memory block reference leak on remove

In the Linux kernel, the following vulnerability has been resolved: mm/memory_hotplug: fix memory block reference leak on remove Patch series "mm: Fix memory block leaks and locking", v2. This series fixes two memory block device reference leaks and one locking issue around the per-memory_block hwpoison counter. This patch (of 2): remove_memory_blocks_and_altmaps() looks up each memory block with find_memory_block(), which acquires a reference to the memory block device. That reference is never dropped on this path, resulting in a leaked device reference when removing memory blocks and their altmaps. Drop the reference after retrieving mem->altmap and clearing mem->altmap, before removing the memory block device.