nfs: Use PTR_ERR_OR_ZERO in 'nfs41_callback_up' function

Use 'PTR_ERR_OR_ZERO()' rather than 'IS_ERR(...) ? PTR_ERR(...) : 0'.

Signed-off-by: Geyslan G. Bem <geyslan@gmail.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>