nvme-tcp: Fix possible race of io_work and direct send

We may send a request (with or without its data) from two paths:

  1. From our I/O context nvme_tcp_io_work which is triggered from:
    - queue_rq
    - r2t reception
    - socket data_ready and write_space callbacks
  2. Directly from queue_rq if the send_list is empty (because we want to
     save the context switch associated with scheduling our io_work).

However, given that now we have the send_mutex, we may run into a race
condition where none of these contexts will send the pending payload to
the controller. Both io_work send path and queue_rq send path
opportunistically attempt to acquire the send_mutex however queue_rq only
attempts to send a single request, and if io_work context fails to
acquire the send_mutex it will complete without rescheduling itself.

The race can trigger with the following sequence:

  1. queue_rq sends request (no incapsule data) and blocks
  2. RX path receives r2t - prepares data PDU to send, adds h2cdata PDU
     to the send_list and schedules io_work
  3. io_work triggers and cannot acquire the send_mutex - because of (1),
     ends without self rescheduling
  4. queue_rq completes the send, and completes

==> no context will send the h2cdata - timeout.

Fix this by having queue_rq sending as much as it can from the send_list
such that if it still has any left, its because the socket buffer is
full and the socket write_space callback will trigger, thus guaranteeing
that a context will be scheduled to send the h2cdata PDU.

Fixes: db5ad6b7 ("nvme-tcp: try to send request in queue_rq context")
Reported-by: Potnuri Bharat Teja <bharat@chelsio.com>
Reported-by: Samuel Jones <sjones@kalrayinc.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Tested-by: Potnuri Bharat Teja <bharat@chelsio.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>