scsi: scsi_dh_alua: Fix memory leak in alua_rtpg()

buff should be freed before returning with SCSI_DH_RETRY in alua_rtpg().

Signed-off-by: tang.junhui <tang.junhui@zte.com.cn>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>