SUNRPC: Fix an Oops in udp_poll()

udp_poll() checks the struct file for the O_NONBLOCK flag, so we must not
call it with a NULL file pointer.

Fixes: 0ffe86f480 ("SUNRPC: Use poll() to fix up the socket requeue races")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

include/linux/sunrpc/xprtsock.h

@@ -26,6 +26,7 @@ struct sock_xprt {
 	 */
 	struct socket *		sock;
 	struct sock *		inet;
+	struct file *		file;
 
 	/*
 	 * State of TCP reply receive