nvme-tcp: try to send request in queue_rq context

Today, nvme-tcp automatically schedules a send request
to a workqueue context, which is 1 more than we'd need
in case the socket buffer is wide open.

However, because we have async send activity (as a result
of r2t, or write_space callbacks), we need to synchronize
sends from possibly multiple contexts (ideally all running
on the same cpu though).

Thus, we only try to send directly from queue_rq in cases:
1. the send_list is empty
2. we can send it synchronously (i.e. not from the RX path)
3. we run on the same cpu as the queue->io_cpu to avoid
   contention on the send operation.

Proposed-by: Mark Wunderlich <[email protected]>
Signed-off-by: Mark Wunderlich <[email protected]>
Signed-off-by: Sagi Grimberg <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

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