nfs41: remove uneeded checks in callback processing

All callback operations have arguments to decode and require processing.
The preprocess_nfs4X_op functions catch unsupported or illegal ops so
decode_args and process_op pointers are always non NULL.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>