NFSv4.1: Don't set up a backchannel if the server didn't agree to do so

If the server doesn't agree to out backchannel setup request, then
don't set one up.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/callback_proc.c

@@ -427,6 +427,8 @@ __be32 nfs4_callback_sequence(struct cb_sequenceargs *args,
 	if (clp == NULL)
 		goto out;
 
+	if (!(clp->cl_session->flags & SESSION4_BACK_CHAN))
+		goto out;
 	tbl = &clp->cl_session->bc_slot_table;
 
 	spin_lock(&tbl->slot_tbl_lock);