mm, highmem: use PKMAP_NR() to calculate an index of pkmap

To calculate an index of pkmap, using PKMAP_NR() is more understandable
and maintainable, so change it.
Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/highmem.c

@@ -99,7 +99,7 @@ struct page *kmap_to_page(void *vaddr)
 	unsigned long addr = (unsigned long)vaddr;
 
 	if (addr >= PKMAP_ADDR(0) && addr < PKMAP_ADDR(LAST_PKMAP)) {
-		int i = (addr - PKMAP_ADDR(0)) >> PAGE_SHIFT;
+		int i = PKMAP_NR(addr);
 		return pte_page(pkmap_page_table[i]);
 	}