nfsd: Migrate the stateid reference into nfs4_preprocess_seqid_op

Allow nfs4_preprocess_seqid_op to take the stateid reference, instead
of having all the callers do so.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>