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: fadd94e0 (bcache: quit dc->writeback_thread when BCACHE_DEV_DETACHING is set)
Signed-off-by: NShan Hai <shan.hai@oracle.com>
Signed-off-by: NColy Li <colyli@suse.de>
Tested-by: NShenghui Wang <shhuiw@foxmail.com>
Cc: stable@vger.kernel.org #4.17+
Signed-off-by: NJens Axboe <axboe@kernel.dk>