NFSD: Hoist status code encoding into XDR encoder functions

The original intent was presumably to reduce code duplication. The
trade-off was:

- No support for an NFSD proc function returning a non-success
  RPC accept_stat value.
- No support for void NFS replies to non-NULL procedures.
- Everyone pays for the deduplication with a few extra conditional
  branches in a hot path.

In addition, nfsd_dispatch() leaves *statp uninitialized in the
success path, unlike svc_generic_dispatch().

Address all of these problems by moving the logic for encoding
the NFS status code into the NFS XDR encoders themselves. Then
update the NFS .pc_func methods to return an RPC accept_stat
value.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4xdr.c

@@ -5167,15 +5167,14 @@ nfs4svc_decode_compoundargs(struct svc_rqst *rqstp, __be32 *p)
 int
 nfs4svc_encode_compoundres(struct svc_rqst *rqstp, __be32 *p)
 {
-	/*
-	 * All that remains is to write the tag and operation count...
-	 */
 	struct nfsd4_compoundres *resp = rqstp->rq_resp;
 	struct xdr_buf *buf = resp->xdr.buf;
 
 	WARN_ON_ONCE(buf->len != buf->head[0].iov_len + buf->page_len +
 				 buf->tail[0].iov_len);
 
+	*p = resp->cstate.status;
+
 	rqstp->rq_next_page = resp->xdr.page_ptr + 1;
 
 	p = resp->tagp;