xen: clean up xen_load_gdt

Makes the logic a bit clearer.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/enlighten.c

@@ -301,10 +301,21 @@ static void xen_load_gdt(const struct desc_ptr *dtr)
 	frames = mcs.args;
 
 	for (f = 0; va < dtr->address + size; va += PAGE_SIZE, f++) {
-		frames[f] = arbitrary_virt_to_mfn((void *)va);
+		int level;
+		pte_t *ptep = lookup_address(va, &level);
+		unsigned long pfn, mfn;
+		void *virt;
+
+		BUG_ON(ptep == NULL);
+
+		pfn = pte_pfn(*ptep);
+		mfn = pfn_to_mfn(pfn);
+		virt = __va(PFN_PHYS(pfn));
+
+		frames[f] = mfn;
 
 		make_lowmem_page_readonly((void *)va);
-		make_lowmem_page_readonly(mfn_to_virt(frames[f]));
+		make_lowmem_page_readonly(virt);
 	}
 
 	MULTI_set_gdt(mcs.mc, frames, size / sizeof(struct desc_struct));