iscsi_data_xmit() (TX worker) is iterating over the queue of new SCSI
commands concurrently with the queue being replenished. Only after the
queue is emptied will we start sending pending DataOut PDUs. That leads to
DataOut timeout on the target side and to connection reinstatement.

Give priority to pending DataOut commands over new commands.

Link: https://lore.kernel.org/r/20220607131953.11584-1-d.bogdanov@yadro.comReviewed-by: Konstantin Shelekhin <k.shelekhin@yadro.com>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>