block: Use PAGE_SECTORS_SHIFT

The bare use of '9' confuses some people.  We also don't need this cast,
since the compiler does exactly that cast for us.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

block/partitions/core.c

@@ -712,8 +712,7 @@ void *read_part_sector(struct parsed_partitions *state, sector_t n, Sector *p)
 		goto out;
 	}
 
-	page = read_mapping_page(mapping,
-			(pgoff_t)(n >> (PAGE_SHIFT - 9)), NULL);
+	page = read_mapping_page(mapping, n >> PAGE_SECTORS_SHIFT, NULL);
 	if (IS_ERR(page))
 		goto out;