arch/x86: Do not access EFI memory map if it is not available

Do not access EFI memory map if it is not available. At least
Xen dom0 EFI implementation does not have an access to it.
Signed-off-by: NDaniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: NMatt Fleming <matt.fleming@intel.com>

arch/x86/platform/efi/efi.c

@@ -946,6 +946,9 @@ u64 efi_mem_attributes(unsigned long phys_addr)
 	efi_memory_desc_t *md;
 	void *p;
 
+	if (!efi_enabled(EFI_MEMMAP))
+		return 0;
+
 	for (p = memmap.map; p < memmap.map_end; p += memmap.desc_size) {
 		md = p;
 		if ((md->phys_addr <= phys_addr) &&