migration: update recv bitmap only on dest vm

We shouldn't update the received bitmap if we're the source VM.  This
fixes a breakage when release-ram is enabled on postcopy.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180723123305.24792-2-peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/ram.c

@@ -2827,8 +2827,15 @@ int ram_discard_range(const char *rbname, uint64_t start, size_t length)
         goto err;
     }
 
-    bitmap_clear(rb->receivedmap, start >> qemu_target_page_bits(),
-                 length >> qemu_target_page_bits());
+    /*
+     * On source VM, we don't need to update the received bitmap since
+     * we don't even have one.
+     */
+    if (rb->receivedmap) {
+        bitmap_clear(rb->receivedmap, start >> qemu_target_page_bits(),
+                     length >> qemu_target_page_bits());
+    }
+
     ret = ram_block_discard_range(rb, start, length);
 
 err: