scsi: target: iscsi: Fix zero tag inside a trace event

target_sequencer_start event is triggered inside target_cmd_init_cdb().
se_cmd.tag is not initialized with ITT at the moment so the event always
prints zero tag.

Link: https://lore.kernel.org/r/20210403215415.95077-1-r.bolshakov@yadro.com
Cc: stable@vger.kernel.org # 5.10+
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>