kernel/kthread.c: kthread_worker: don't hog the cpu

kernel/kthread.c: kthread_worker: don't hog the cpu

If the worker thread continues getting work, it will hog the cpu and rcu
stall complains.  Make it a good citizen.  This is triggered in a loop
block device test.

Link: http://lkml.kernel.org/r/5de0a179b3184e1a2183fc503448b0269f24d75b.1503697127.git.shli@fb.com
Signed-off-by: Shaohua Li <[email protected]>
Cc: Petr Mladek <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Ingo Molnar <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>