sunrpc: rpc_pipe_poll may miss available data in some cases

Pipe messages start out life on a queue on the inode, but when first
read they're moved to the filp's private pointer.  So it's possible for
a poll here to return null even though there's a partially read message
available.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/rpc_pipe.c

@@ -280,7 +280,7 @@ rpc_pipe_poll(struct file *filp, struct poll_table_struct *wait)
 	mask = POLLOUT | POLLWRNORM;
 	if (rpci->ops == NULL)
 		mask |= POLLERR | POLLHUP;
-	if (!list_empty(&rpci->pipe))
+	if (filp->private_data || !list_empty(&rpci->pipe))
 		mask |= POLLIN | POLLRDNORM;
 	return mask;
 }