nfsd4: centralize handling of replay owners

Set the stateowner associated with a replay in one spot in
nfs4_preprocess_seqid_op() and keep it in cstate.  This allows removing
a few lines of boilerplate from all the nfs4_preprocess_seqid_op()
callers.

Also turn ENCODE_SEQID_OP_TAIL into a function while we're here.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>