fuse: set FR_SENT while locked

Otherwise fuse_dev_do_write() could come in and finish off the request, and
the set_bit(FR_SENT, ...) could trigger the WARN_ON(test_bit(FR_SENT, ...))
in request_end().
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reported-by: syzbot+ef054c4d3f64cd7f7cec@syzkaller.appspotmai
Fixes: 46c34a34 ("fuse: no fc->lock for pqueue parts")
Cc: <stable@vger.kernel.org> # v4.2

fs/fuse/dev.c

@@ -1312,8 +1312,8 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
 	}
 	list_move_tail(&req->list, &fpq->processing);
 	__fuse_get_request(req);
-	spin_unlock(&fpq->lock);
 	set_bit(FR_SENT, &req->flags);
+	spin_unlock(&fpq->lock);
 	/* matches barrier in request_wait_answer() */
 	smp_mb__after_atomic();
 	if (test_bit(FR_INTERRUPTED, &req->flags))