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 <taochiu@synology.com>
Signed-off-by: Cody Wong <codywong@synology.com>
Reviewed-by: Leon Chien <leonchien@synology.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>