xen/balloon: Use PageHighMem() for high memory page detection

Replace pfn < max_low_pfn by !PageHighMem() in increase_reservation().
It makes more clearer what is going on.
Acked-by: NIan Campbell <ian.campbell@citrix.com>
Acked-by: NDaniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: NDaniel Kiper <dkiper@net-space.pl>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/balloon.c

@@ -246,7 +246,7 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
 		set_phys_to_machine(pfn, frame_list[i]);
 
 		/* Link back into the page tables if not highmem. */
-		if (!xen_hvm_domain() && pfn < max_low_pfn) {
+		if (!xen_hvm_domain() && !PageHighMem(page)) {
 			int ret;
 			ret = HYPERVISOR_update_va_mapping(
 				(unsigned long)__va(pfn << PAGE_SHIFT),