scsi: qla2xxx: Fix stale mem access on driver unload

On driver unload, 'remove_one' thread was allowed to advance, while session
cleanup still lag behind.  This patch ensures session deletion will finish
before remove_one can advance.

Link: https://lore.kernel.org/r/20190912180918.6436-4-hmadhani@marvell.com
Signed-off-by: Quinn Tran <qutran@marvell.com>
Signed-off-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>