x86-64, NUMA: Do not scan two times for setup_node_bootmem()

By the time setup_node_bootmem() is called, all the memblocks are
already registered.  As node_data is allocated from these memblocks,
calling it more than once doesn't make any difference.  Drop the loop.

tj: Dropped comment referencing to the old behavior as suggested by
    David and rephrased the description.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>