fuse: check connected before queueing on fpq->io

A request could end up on the fpq->io list after fuse_abort_conn() has
reset fpq->connected and aborted requests on that list:

Thread-1			  Thread-2
========			  ========
->fuse_simple_request()           ->shutdown
  ->__fuse_request_send()
    ->queue_request()		->fuse_abort_conn()
->fuse_dev_do_read()                ->acquire(fpq->lock)
  ->wait_for(fpq->lock) 	  ->set err to all req's in fpq->io
				  ->release(fpq->lock)
  ->acquire(fpq->lock)
  ->add req to fpq->io

After the userspace copy is done the request will be ended, but
req->out.h.error will remain uninitialized.  Also the copy might block
despite being already aborted.

Fix both issues by not allowing the request to be queued on the fpq->io
list after fuse_abort_conn() has processed this list.
Reported-by: NPradeep P V K <pragalla@codeaurora.org>
Fixes: fd22d62e ("fuse: no fc->lock for iqueue parts")
Cc: <stable@vger.kernel.org> # v4.2
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

fs/fuse/dev.c

@@ -1272,6 +1272,15 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
 		goto restart;
 	}
 	spin_lock(&fpq->lock);
+	/*
+	 *  Must not put request on fpq->io queue after having been shut down by
+	 *  fuse_abort_conn()
+	 */
+	if (!fpq->connected) {
+		req->out.h.error = err = -ECONNABORTED;
+		goto out_end;
+
+	}
 	list_add(&req->list, &fpq->io);
 	spin_unlock(&fpq->lock);
 	cs->req = req;