nvme-pci: fix controller reset hang when racing with nvme_timeout

reset_work() in nvme-pci may hang forever in the following scenario:
1) A reset caused by a command timeout occurs due to a controller being
temporarily irresponsive.
2) nvme_reset_work() restarts admin queue at nvme_alloc_admin_tags(). At
the same time, a user-submitted admin command is queued and waiting
for completion. Then, reset_work() changes its state to CONNECTING,
and submits an identify command.
3) However, the controller does still not respond to any command,
causing a timeout being fired at the user-submitted command.
Unfortunately, nvme_timeout() does not see the completion on cq, and
any timeout that takes place under CONNECTING state causes a
controller shutdown.
4) Normally, the identify command in reset_work() would be canceled with
SC_HOST_ABORTED by nvme_dev_disable(), then reset_work can tear down
the controller accordingly. But the controller happens to return
online and respond the identify command before nvme_dev_disable()
should have been reaped it off.
5) reset_work() continues to setup_io_queues() as it observes no error
in init_identify(). However, the admin queue has already been
quiesced in dev_disable(). Thus, any following commands would be
blocked forever in blk_execute_rq().

This can be fixed by restricting usercmd commands when controller is not
in a LIVE state in nvme_queue_rq(), as what has been done previously in
fabrics.

```
nvme_reset_work(): |
nvme_alloc_admin_tags() |
| nvme_submit_user_cmd():
nvme_init_identify(): | ...
__nvme_submit_sync_cmd(): |
... | ...
---------------------------------------> nvme_timeout():
(Controller starts reponding commands) | nvme_dev_disable(, true):
nvme_setup_io_queues(): |
__nvme_submit_sync_cmd(): |
(hung in blk_execute_rq |
since run_hw_queue sees |
queue quiesced) |

```

Signed-off-by: Tao Chiu <[e-mail]>
Signed-off-by: Cody Wong <[e-mail]>
Reviewed-by: Leon Chien <[e-mail]>
Reviewed-by: Keith Busch <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>

drivers/nvme/host/pci.c

···
 	if (unlikely(!test_bit(NVMEQ_ENABLED, &nvmeq->flags)))
 		return BLK_STS_IOERR;
 
+	if (!nvme_check_ready(&dev->ctrl, req, true))
+		return nvme_fail_nonready_command(&dev->ctrl, req);
+
 	ret = nvme_setup_cmd(ns, req);
 	if (ret)
 		return ret;