According to 'admin-guide/mm/memory-hotplug.rst', the memory block ID,
instead of the section index, is shown by '/sys/devices/system/memory/
memoryX/phys_index'.
Fix the comments to match with 'admin-guide/mm/memory-hotplug.rst'.
Besides, use the existing helper memory_block_id() to convert the section
index to the memory block index.
No functional change intended.
Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
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