memory-hotplug: remove redundant call of page_to_pfn

This is just a small optimization.  The start_pfn can be obtained directly
by phys_index << PFN_SECTION_SHIFT.  So the call of page_to_pfn() is
redundant and remove it.
Signed-off-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Acked-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Wang Nan <wangnan0@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/base/memory.c

@@ -228,8 +228,8 @@ memory_block_action(unsigned long phys_index, unsigned long action, int online_t
 	struct page *first_page;
 	int ret;
 
-	first_page = pfn_to_page(phys_index << PFN_SECTION_SHIFT);
-	start_pfn = page_to_pfn(first_page);
+	start_pfn = phys_index << PFN_SECTION_SHIFT;
+	first_page = pfn_to_page(start_pfn);
 
 	switch (action) {
 		case MEM_ONLINE: