NFS: kmalloc() doesn't return an ERR_PTR()

Obviously we should check for NULL here instead of IS_ERR().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable@vger.kernel.org [3.4]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>