xen: make sure xen_extra_mem_start is beyond all non-RAM e820

If Xen gives us non-RAM E820 entries (dom0 only, typically), then
make sure the extra RAM region is beyond them.  It's OK for
the extra space to grow into E820 regions, however.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/setup.c

@@ -167,7 +167,8 @@ char * __init xen_memory_setup(void)
 
 				extra_pages += PFN_DOWN(end - mem_end);
 			}
-		}
+		} else if (map[i].type != E820_RAM)
+			xen_extra_mem_start = end;
 		if (map[i].size > 0)
 			e820_add_region(map[i].addr, map[i].size, map[i].type);
 	}