SUNRPC: IS_ERR/PTR_ERR confusion

IS_ERR returns 1 or 0, PTR_ERR returns the error value.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>