scsi: target: Simplify transport_generic_free_cmd() (2/2)

Since target_wait_free_cmd() skips TMFs with no associated LUN, it is safe
to call that function for such commands. Use this to simplify
transport_generic_free_cmd(). The only functional change in this patch is
that CMD_T_FABRIC_STOP gets set for TMFs with no associated LUN by
transport_generic_free_cmd().

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>