scsi: target: alua: fix the tg_pt_gps_count

Reducing the count should be alua_tg_pt_gps_count instead of
alua_tg_pt_gps_counter when free alua group.

Signed-off-by: tangwenji <tang.wenji@zte.com.cn>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>