scsi: scsi_transport_iscsi: Use flush_work in iscsi_remove_session

scsi_flush_work flushes workqueue for the Scsi_Host.  In iSCSI offload
enabled host, this would wait for all other sessions under the host.

Use flush_work for the session being removed instead.

Signed-off-by: Jitendra Bhivare <jitendra.bhivare@broadcom.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Reviewed-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>