drivers/base/node.c: simplify unregister_memory_block_under_nodes()

drivers/base/node.c: simplify unregister_memory_block_under_nodes()

We don't allow to offline memory block devices that belong to multiple
numa nodes.  Therefore, such devices can never get removed.  It is
sufficient to process a single node when removing the memory block.  No
need to iterate over each and every PFN.

We already have the nid stored for each memory block.  Make sure that the
nid always has a sane value.

Please note that checking for node_online(nid) is not required.  If we
would have a memory block belonging to a node that is no longer offline,
then we would have a BUG in the node offlining code.

Link: http://lkml.kernel.org/r/20190719135244.15242-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>