bcache: fix uninterruptible sleep in writeback thread

There were two issues here:

- writeback thread did not start until the device first became dirty
- writeback thread used uninterruptible sleep once running

Without this patch I see kernel warnings printed and a load average of
1.52 after booting my test VM. With this patch the warnings are gone and
the load average is near 0.00 as expected.

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

drivers/md/bcache/super.c

@@ -1042,6 +1042,9 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
 	 */
 	atomic_set(&dc->count, 1);
 
+	if (bch_cached_dev_writeback_start(dc))
+		return -ENOMEM;
+
 	if (BDEV_STATE(&dc->sb) == BDEV_STATE_DIRTY) {
 		bch_sectors_dirty_init(dc);
 		atomic_set(&dc->has_dirty, 1);