x86: fix WARN_ON() message: teach page_is_ram() about the special 4Kb bios data page

This patch teaches page_is_ram() about the fact that the first
4Kb of memory are special on x86, even though the E820 table
normally doesn't exclude it.

This fixes the WARN_ON() reported by Laurent Riffard who was also
very helpful in diagnosing the issue.

[ mingo@elte.hu: we are working on doing this properly in the e820
  space, but for 2.6.25 this is the better fix. ]

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>