md/raid5: release/flush io in raid5_do_work()

In raid5, there are scenarios where some ios are deferred to a later
time, and some IO need a flush to complete. To make sure we make
progress with these IOs, we need to call the following functions:

    flush_deferred_bios(conf);
    r5l_flush_stripe_to_raid(conf->log);

Both of these functions are called in raid5d(), but missing in
raid5_do_work(). As a result, these functions are not called
when multi-threading (group_thread_cnt > 0) is enabled. This patch
adds calls to these function to raid5_do_work().

Note for stable branches:

  r5l_flush_stripe_to_raid(conf->log) is need for 4.4+
  flush_deferred_bios(conf) is only needed for 4.11+

Cc: stable@vger.kernel.org (4.4+)
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid5.c

@@ -6235,6 +6235,10 @@ static void raid5_do_work(struct work_struct *work)
 
 	spin_unlock_irq(&conf->device_lock);
 
+	flush_deferred_bios(conf);
+
+	r5l_flush_stripe_to_raid(conf->log);
+
 	async_tx_issue_pending_all();
 	blk_finish_plug(&plug);