Call to set_dirty_tracking() is misplaced.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: qemu/block-migration.c
===================================================================
--- qemu.orig/block-migration.c
+++ qemu/block-migration.c
@@ -528,6 +528,8 @@ static void blk_mig_cleanup(Monitor *mon
     BlkMigDevState *bmds;
     BlkMigBlock *blk;
 
+    set_dirty_tracking(0);
+
     while ((bmds = QSIMPLEQ_FIRST(&block_mig_state.bmds_list)) != NULL) {
         QSIMPLEQ_REMOVE_HEAD(&block_mig_state.bmds_list, entry);
         qemu_free(bmds->aio_bitmap);
@@ -540,8 +542,6 @@ static void blk_mig_cleanup(Monitor *mon
         qemu_free(blk);
     }
 
-    set_dirty_tracking(0);
-
     monitor_printf(mon, "\n");
 }