blk-mq: fix io hung due to missing commit_rqs

commit 65fac0d5 upstream.

Currently, in virtio_scsi, if 'bd->last' is not set to true while
dispatching request, such io will stay in driver's queue, and driver
will wait for block layer to dispatch more rqs. However, if block
layer failed to dispatch more rq, it should trigger commit_rqs to
inform driver.

There is a problem in blk_mq_try_issue_list_directly() that commit_rqs
won't be called:

// assume that queue_depth is set to 1, list contains two rq
blk_mq_try_issue_list_directly
 blk_mq_request_issue_directly
 // dispatch first rq
 // last is false
  __blk_mq_try_issue_directly
   blk_mq_get_dispatch_budget
   // succeed to get first budget
   __blk_mq_issue_directly
    scsi_queue_rq
     cmd->flags |= SCMD_LAST
      virtscsi_queuecommand
       kick = (sc->flags & SCMD_LAST) != 0
       // kick is false, first rq won't issue to disk
 queued++

 blk_mq_request_issue_directly
 // dispatch second rq
  __blk_mq_try_issue_d...