xfs: check kthread_should_stop() after the setting of task state

A umount hang is possible when a race occurs between the umount
process and the xfsaild kthread. The following sequences outline
the race:

    xfsaild: kthread_should_stop()
	     => return false, so xfsaild continue

    umount: set_bit(KTHREAD_SHOULD_STOP, &kthread->flags)
	    => by kthread_stop()
    umount: wake_up_process()
	    => because xfsaild is still running, so 0 is returned

    xfsaild: __set_current_state(TASK_INTERRUPTIBLE)
    xfsaild: schedule()
	    => now, xfsaild will wait indefinitely

    umount: wait_for_completion()
	    => and umount will hang

To fix that, we need to check kthread_should_stop() after we set
the task state, so the xfsaild will either see the stop bit and
exit or the task state is reset to runnable by wake_up_process()
such that it isn't scheduled out indefinitely and detects the stop
bit at the next iteration.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>