bcache: release dc->writeback_lock properly in bch_writeback_thread()

The writeback thread would exit with a lock held when the cache device
is detached via sysfs interface, fix it by releasing the held lock
before exiting the while-loop.

Fixes: fadd94e (bcache: quit dc->writeback_thread when BCACHE_DEV_DETACHING is set)
Signed-off-by: Shan Hai <[email protected]>
Signed-off-by: Coly Li <[email protected]>
Tested-by: Shenghui Wang <[email protected]>
Cc: [email protected] #4.17+
Signed-off-by: Jens Axboe <[email protected]>
(cherry picked from commit 3943b04)

Orabug: 28335202

Signed-off-by: Shan Hai <[email protected]>
Reviewed-by: Darren Kenny <[email protected]>

Author: Shan Hai

drivers/md/bcache/writeback.c

@@ -455,8 +455,10 @@ static int bch_writeback_thread(void *arg)
 			 * data on cache. BCACHE_DEV_DETACHING flag is set in
 			 * bch_cached_dev_detach().
 			 */
-			if (test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags))
+			if (test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags)) {
+				up_write(&dc->writeback_lock);
 				break;
+			}
 		}
 
 		up_write(&dc->writeback_lock);