drivers/base/memory: Fix comments for phys_index_show()

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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/memory.c

···
 }
 EXPORT_SYMBOL_GPL(memory_block_size_bytes);
 
-/*
- * Show the first physical section index (number) of this memory block.
- */
+/* Show the memory block ID, relative to the memory block size */
 static ssize_t phys_index_show(struct device *dev,
 			       struct device_attribute *attr, char *buf)
 {
 	struct memory_block *mem = to_memory_block(dev);
-	unsigned long phys_index;
 
-	phys_index = mem->start_section_nr / sections_per_block;
-
-	return sysfs_emit(buf, "%08lx\n", phys_index);
+	return sysfs_emit(buf, "%08lx\n", memory_block_id(mem->start_section_nr));
 }
 
 /*