nfsd: Fix an Oops in free_session()

In call_xpt_users(), we delete the entry from the list, but we
do not reinitialise it. This triggers the list poisoning when
we later call unregister_xpt_user() in nfsd4_del_conns().
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/svc_xprt.c

@@ -989,7 +989,7 @@ static void call_xpt_users(struct svc_xprt *xprt)
 	spin_lock(&xprt->xpt_lock);
 	while (!list_empty(&xprt->xpt_users)) {
 		u = list_first_entry(&xprt->xpt_users, struct svc_xpt_user, list);
-		list_del(&u->list);
+		list_del_init(&u->list);
 		u->callback(u);
 	}
 	spin_unlock(&xprt->xpt_lock);