NFS: remove error field from nfs_readdir_descriptor_t

The error field in nfs_readdir_descriptor_t is never used outside of the
function in which it is set. Remove the field and change the place that
does use it to use an existing local variable.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>