nvme-tcp: fix io_work priority inversion

Dispatching requests inline with the .queue_rq() call may block while
holding the send_mutex. If the tcp io_work also happens to schedule, it
may see the req_list is non-empty, leaving "pending" true and remaining
in TASK_RUNNING. Since io_work is of higher scheduling priority, the
.queue_rq task may not get a chance to run, blocking forward progress
and leading to io timeouts.

Instead of checking for pending requests within io_work, let the queueing
restart io_work outside the send_mutex lock if there is more work to be
done.

Fixes: a0fdd141

 ("nvme-tcp: rerun io_work if req_list is not empty")
Reported-by: Samuel Jones <sjones@kalrayinc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>