Btrfs: Check kthread_should_stop() before schedule() in worker_loop

In worker_loop(), the func should check whether it has been requested to stop
before it decides to schedule out.

Otherwise if the stop request(also the last wake_up()) sent by
btrfs_stop_workers() happens when worker_loop() running after the "while"
judgement and before schedule(), woker_loop() will schedule away and never be
woken up, which will also cause btrfs_stop_workers() wait forever.

Signed-off-by: Chris Mason <chris.mason@oracle.com>