migration: Make dirty_pages_rate atomic

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>

---

Don't use __nocheck() variants
Use stat64_get()
diff --git a/migration/ram.c b/migration/ram.c
index e9dcda8..5846f6e 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1130,8 +1130,9 @@
     double compressed_size;
 
     /* calculate period counters */
-    ram_counters.dirty_pages_rate = rs->num_dirty_pages_period * 1000
-                / (end_time - rs->time_last_bitmap_sync);
+    stat64_set(&ram_counters.dirty_pages_rate,
+               rs->num_dirty_pages_period * 1000 /
+               (end_time - rs->time_last_bitmap_sync));
 
     if (!page_count) {
         return;