scsi: qla2xxx: Serialize session deletion by using work_lock

for session deletion, replace sess_lock with work_lock.
Under certain case sess_lock is not feasiable to acquire.
The lock is needed temporarily to make sure a single
call to schedule of the work element.

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>