MDEV-28731 Race condition on log checkpoint

mtr_t::modify(): Set the m_made_dirty flag if needed,
so that buf_pool_t::insert_into_flush_list() will be invoked
while holding log_sys.flush_order_mutex.

This is something that was should have been part of
commit b212f1da (MDEV-22107).

storage/innobase/mtr/mtr0mtr.cc

@@ -1263,4 +1263,6 @@ void mtr_t::modify(const buf_block_t &block)
   }
   iteration.functor.found->type= static_cast<mtr_memo_type_t>
     (iteration.functor.found->type | MTR_MEMO_MODIFY);
+  if (is_block_dirtied(&block))
+    m_made_dirty= true;
 }