scsi: qla2xxx: Always wait for qlt_sess_work_fn() from qlt_stop_phase1()

scsi: qla2xxx: Always wait for qlt_sess_work_fn() from qlt_stop_phase1()

Currently qlt_stop_phase1() may fail to call flush_scheduled_work(), for
list_empty() may return true as soon as qlt_sess_work_fn() called
list_del(). In order to close this race window, check list_empty() after
calling flush_scheduled_work().

If this patch causes problems, please check commit c4f135d64382
("workqueue: Wrap flush_workqueue() using a macro"). We are on the way to
remove all flush_scheduled_work() calls from the kernel.

Link: https://lore.kernel.org/r/7f24469d-9e39-3398-d851-329b54c0b923@I-love.SAKURA.ne.jp
Tested-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>