x86: ioremap ram check fix

bdd3cee2 (x86: ioremap(), extend check
to all RAM pages) breaks OLPC's ioremap call.  The ioremap that OLPC uses is:

        romsig = ioremap(0xffffffc0, 16);

The commit that breaks it is basically:

-       for (pfn = phys_addr >> PAGE_SHIFT; pfn < max_pfn_mapped &&
-            (pfn << PAGE_SHIFT) < last_addr; pfn++) {
+       for (pfn = phys_addr >> PAGE_SHIFT;
+                               (pfn << PAGE_SHIFT) < last_addr; pfn++) {
+

Previously, the 'pfn < max_pfn_mapped' check would've caused us to not
enter the loop.  Removing that check means we loop infinitely.  The
reason for that is because pfn is 0xfffff, and last_addr is 0xffffffcf.
The remaining check that is used to exit the loop is not sufficient;
when pfn<<PAGE_SHIFT is 0xfffff000, that is less than 0xffffffcf; when
we increment pfn and it overflows (pfn == 0x100000), pfn<<PAGE_SHIFT
ends up being 0.  That, of course, is less than last_addr.  In effect,
pfn<<PAGE_SHIFT is never lower than last_addr.

The simple fix for this is to limit the last_addr check to the PAGE_MASK;
a patch is below.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>