x86: fix page_is_ram() thinko

page_is_ram() has a special case for the 640k-1M bios area, however
due to a thinko the special case checks the e820 table entry and
not the memory the user has asked for. This patch fixes the bug.

[ mingo@elte.hu: this too is better solved in the e820 space, but those
  fixes are too intrusive for v2.6.25. ]

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>