migration/postcopy: wait for decompress thread in precopy

Compress is not supported with postcopy, it is safe to wait for
decompress thread just in precopy.

This is a preparation for later patch.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/ram.c

@@ -4421,6 +4421,7 @@ static int ram_load_precopy(QEMUFile *f)
         }
     }
 
+    ret |= wait_for_decompress_done();
     return ret;
 }
 
@@ -4452,8 +4453,6 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
         } else {
             ret = ram_load_precopy(f);
         }
-
-        ret |= wait_for_decompress_done();
     }
     trace_ram_load_complete(ret, seq_iter);