blk-mq: avoid infinite recursion with the FUA flag

We should not insert requests into the flush state machine from
blk_mq_insert_request.  All incoming flush requests come through
blk_{m,s}q_make_request and are handled there, while blk_execute_rq_nowait
should only be called for BLOCK_PC requests.  All other callers
deal with requests that already went through the flush statemchine
and shouldn't be reinserted into it.

Reported-by: Robert Elliott  <Elliott@hp.com>
Debugged-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-exec.c

@@ -56,6 +56,7 @@ void blk_execute_rq_nowait(struct request_queue *q, struct gendisk *bd_disk,
 	bool is_pm_resume;
 
 	WARN_ON(irqs_disabled());
+	WARN_ON(rq->cmd_type == REQ_TYPE_FS);
 
 	rq->rq_disk = bd_disk;
 	rq->end_io = done;