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>
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 006bfa3..5985379 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -7195,6 +7195,8 @@
 	struct nfs4_channel_attrs *sent = &args->bc_attrs;
 	struct nfs4_channel_attrs *rcvd = &res->bc_attrs;
 
+	if (!(res->flags & SESSION4_BACK_CHAN))
+		goto out;
 	if (rcvd->max_rqst_sz > sent->max_rqst_sz)
 		return -EINVAL;
 	if (rcvd->max_resp_sz < sent->max_resp_sz)
@@ -7206,6 +7208,7 @@
 		return -EINVAL;
 	if (rcvd->max_reqs != sent->max_reqs)
 		return -EINVAL;
+out:
 	return 0;
 }
 
@@ -7226,7 +7229,9 @@
 	nfs4_copy_sessionid(&session->sess_id, &res->sessionid);
 	session->flags = res->flags;
 	memcpy(&session->fc_attrs, &res->fc_attrs, sizeof(session->fc_attrs));
-	memcpy(&session->bc_attrs, &res->bc_attrs, sizeof(session->bc_attrs));
+	if (res->flags & SESSION4_BACK_CHAN)
+		memcpy(&session->bc_attrs, &res->bc_attrs,
+				sizeof(session->bc_attrs));
 }
 
 static int _nfs4_proc_create_session(struct nfs_client *clp,