NFS: If nfs_mountpoint_expiry_timeout < 0, do not expire submounts

If we set nfs_mountpoint_expiry_timeout to a negative value, then
allow that to imply that we do not expire NFSv4 submounts.
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/namespace.c

@@ -157,6 +157,9 @@ struct vfsmount *nfs_d_automount(struct path *path)
 	if (IS_ERR(mnt))
 		goto out;
 
+	if (nfs_mountpoint_expiry_timeout < 0)
+		goto out;
+
 	mntget(mnt); /* prevent immediate expiration */
 	mnt_set_expiry(mnt, &nfs_automount_list);
 	schedule_delayed_work(&nfs_automount_task, nfs_mountpoint_expiry_timeout);