scsi: qla2xxx: Reduce redundant ADISC command for RSCNs

For each RSCN that triggers a rescan of the fabric, ADISC is used to
revalidate an existing session. If the RSCN is not affecting all
existing sessions, then driver should not send redundant ADISC for all
existing sessions.

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>