mm/page_alloc.c: extract the common part in pfn_to_bitidx()

The return value calculation is the same both for SPARSEMEM or not.

Just take it out.
Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Link: http://lkml.kernel.org/r/20200623124201.8199-2-richard.weiyang@linux.alibaba.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -459,11 +459,10 @@ static inline int pfn_to_bitidx(struct page *page, unsigned long pfn)
 {
 #ifdef CONFIG_SPARSEMEM
 	pfn &= (PAGES_PER_SECTION-1);
-	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
 #else
 	pfn = pfn - round_down(page_zone(page)->zone_start_pfn, pageblock_nr_pages);
-	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
 #endif /* CONFIG_SPARSEMEM */
+	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
 }
 
 /**