rgw: log unlink_instance mtime as object's mtime

This is needed so that when squashing entries during data
sync we get a consistent mtime for that object.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>

src/cls/rgw/cls_rgw.cc

@@ -1673,7 +1673,7 @@ static int rgw_bucket_unlink_instance(cls_method_context_t hctx, bufferlist *in,
     rgw_bucket_entry_ver ver;
     ver.epoch = (op.olh_epoch ? op.olh_epoch : olh.get_epoch());
 
-    real_time mtime = real_clock::now(); /* mtime has no real meaning in instance removal context */
+    real_time mtime = obj.mtime(); /* mtime has no real meaning in instance removal context */
     ret = log_index_operation(hctx, op.key, CLS_RGW_OP_UNLINK_INSTANCE, op.op_tag,
                               mtime, ver,
                               CLS_RGW_STATE_COMPLETE, header.ver, header.max_marker,