NVMe: fix flush handling to work with requests
NVMe: remove obsolete todo
NVMe: querying requests always have a valid nvme queue.

The request queue will only send requests, after the nvme queues have
been initialized. Therefore, lets not check for it anymore in the
queue request path.
NVMe: Move tag initialization outside ns alloc

Mistakenly reinitialized the tags for each ns. Move outside to only be
initialized once (and reused multiple times as intended).
NVMe: return busy on iod failure

Jens pointed out that we'll receive -EIO buffer errors on low memory
conditions. Instead we want to tell blk-mq to wait.