writeback: Fix lost wake-up shutting down writeback thread

Setting the task state here may cause us to miss the wake up from
kthread_stop(), so we need to recheck kthread_should_stop() or risk
sleeping forever in the following schedule().

Symptom was an indefinite hang on an NFSv4 mount.  (NFSv4 may create
multiple mounts in a temporary namespace while traversing the mount
path, and since the temporary namespace is immediately destroyed, it may
end up destroying a mount very soon after it was created, possibly
making this race more likely.)

INFO: task mount.nfs4:4314 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
mount.nfs4    D 0000000000000000  2880  4314   4313 0x00000000
 ffff88001ed6da28 0000000000000046 ffff88001ed6dfd8 ffff88001ed6dfd8
 ffff88001ed6c000 ffff88001ed6c000 ffff88001ed6c000 ffff88001e5003a0
 ffff88001ed6dfd8 ffff88001e5003a8 ffff88001ed6c000 ffff88001ed6dfd8
Call Trace:
 [<ffffffff8196090d>] schedule_timeout+0x1cd/0x2e0
 [...