linux-aio: explain why max batch is checked in laio_io_unplug()

It may not be obvious why laio_io_unplug() checks max batch. I discussed
this with Stefano and have added a comment summarizing the reason.

Cc: Stefano Garzarella <sgarzare@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20220609164712.1539045-3-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/linux-aio.c

@@ -365,6 +365,12 @@ void laio_io_unplug(BlockDriverState *bs, LinuxAioState *s,
     assert(s->io_q.plugged);
     s->io_q.plugged--;
 
+    /*
+     * Why max batch checking is performed here:
+     * Another BDS may have queued requests with a higher dev_max_batch and
+     * therefore in_queue could now exceed our dev_max_batch. Re-check the max
+     * batch so we can honor our device's dev_max_batch.
+     */
     if (s->io_q.in_queue >= laio_max_batch(s, dev_max_batch) ||
         (!s->io_q.plugged &&
          !s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending))) {