fix compat memory exchange op splitting

A shift with a negative count was erroneously used here, yielding
undefined behavior.

Reported-by: Xi Wang <xi@xxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- 2013-02-21.orig/xen/common/compat/memory.c	2013-02-26 17:22:57.000000000 +0100
+++ 2013-02-21/xen/common/compat/memory.c	2013-03-01 13:01:22.000000000 +0100
@@ -173,7 +173,7 @@ int compat_memory_op(unsigned int cmd, X
                 if ( order_delta >= 0 )
                     nat.xchg->out.nr_extents = end_extent >> order_delta;
                 else
-                    nat.xchg->out.nr_extents = end_extent << order_delta;
+                    nat.xchg->out.nr_extents = end_extent << -order_delta;
                 ++split;
             }