When I studied the code of mm/swap, I found "1 << (PAGE_SHIFT - 9)" appears
many times. So I try to clean up it.
1. Replace "1 << (PAGE_SHIFT - 9)" or similar with SECTORS_PER_PAGE
2. Replace "PAGE_SHIFT - 9" with SECTORS_PER_PAGE_SHIFT
3. Replace "9" with SECTOR_SHIFT
4. Replace "512" with SECTOR_SIZE