scsi: qla2xxx: Serialize session free in qlt_free_session_done

Add free_pending flag to serialize queueing of
free_work element onto the work queue

Signed-off-by: Quinn Tran <quinn.tran@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>