bcache: Use uninterruptible sleep in writeback
bcache: Use uninterruptible sleep in writeback

We're just waiting on kthread_should_stop(), nothing else, so
interruptible sleep was wrong here.

Signed-off-by: Kent Overstreet <99dffa8a9668ab7f6cde1c94b42a210109971113@daterainc.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs