NVMe: Fix check_flush_dependency warning

If the controller fails and is degraded after a reset, we need to kill
off all requests queues before removing the inaccessble namespaces. This
will prevent del_gendisk from syncing dirty data, which we can't due
from a WQ_MEM_RECLAIM work queue.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>