[PATCH resend] bcache: prevent crash on changing writeback_running

[Stefan Bader]

I sent this out quite a while ago. Back then it was said that it
was applied to the development tree. But it seems it never made its
way upstream. 
I re-tested with a 4.0.7 kernel and compared this against a 4.0 kernel
with this patch applied. Still it is possible to trigger a NULL pointer
violation when writing 0 into writeback_running of a bcache device that
is not attached to a cache set (without the patch).

-Stefan


---