f2fs: wake up all waiters in f2fs_submit_discard_endio

There could be more than one waiter waiting discard IO completion, so we
need use complete_all() instead of complete() in f2fs_submit_discard_endio
to avoid hungtask.

Fixes: 	ec9895ad ("f2fs: don't hold cmd_lock during waiting discard
command")
Cc: <stable@vger.kernel.org>
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -752,7 +752,7 @@ static void f2fs_submit_discard_endio(struct bio *bio)
 
 	dc->error = bio->bi_error;
 	dc->state = D_DONE;
-	complete(&dc->wait);
+	complete_all(&dc->wait);
 	bio_put(bio);
 }