tools/vm/page-types.c: support swap entry

/proc/pid/pagemap (pte_to_pagemap_entry() internally) already reports
about swap entry, so let's make the in-kernel utility aware of it.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>