nfsd: stop pretending that we cache the SEQUENCE reply.

nfsd: stop pretending that we cache the SEQUENCE reply.

nfsd does not cache the reply to a SEQUENCE.  As the comment above
nfsd4_replay_cache_entry() says:

 * The sequence operation is not cached because we can use the slot and
 * session values.

The comment above nfsd4_cache_this() suggests otherwise.

 * The session reply cache only needs to cache replies that the client
 * actually asked us to.  But it's almost free for us to cache compounds
 * consisting of only a SEQUENCE op, so we may as well cache those too.
 * Also, the protocol doesn't give us a convenient response in the case
 * of a replay of a solo SEQUENCE op that wasn't cached

The code in nfsd4_store_cache_entry() makes it clear that only responses
beyond 'cstate.data_offset' are actually cached, and data_offset is set
at the end of nfsd4_encode_sequence() *after* the sequence response has
been encoded.

This patch simplifies code and removes the confusing comments.

- nfsd4_is_solo_sequence() is discarded as not-useful.
- nfsd4_cache_this() is now trivial so it too is discarded with the
  code placed in-line at the one call-site in nfsd4_store_cache_entry().
- nfsd4_enc_sequence_replay() is open-coded in to
  nfsd4_replay_cache_entry(), and then simplified to (hopefully) make
  the process of replaying a reply clearer.

Signed-off-by: NeilBrown <neil@brown.name>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>