powerpc: Oops in pseries_lmb_remove()

Testing hotplug memory remove has revealed that we can oops in
pseries_lmb_remove().  The incorrect shift causes a NULL pointer
dereference in the page_zone() inline routine.

I have only been able to reproduce the oops on kernels with large pages
enabled.

Tested on Power5 and Power6 with and without large pages enabled.
Signed-off-by: NNathan Fontenot <nfont@austin.ibm.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/hotplug-memory.c

@@ -21,7 +21,7 @@ static int pseries_remove_lmb(unsigned long base, unsigned int lmb_size)
 	struct zone *zone;
 	int ret;
 
-	start_pfn = base >> PFN_SECTION_SHIFT;
+	start_pfn = base >> PAGE_SHIFT;
 	zone = page_zone(pfn_to_page(start_pfn));
 
 	/*