nfsd: don't assume copy notify when preprocessing the stateid

Move the stateid handling to nfsd4_copy_notify.
If nfs4_preprocess_stateid_op did not produce an output stateid, error out.

Copy notify specifically does not permit the use of special stateids,
so enforce that outside generic stateid pre-processing.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Olga Kornievskaia <aglo@umich.edu>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>