NFSD: Save location of NFSv4 COMPOUND status

Refactor: Currently nfs4svc_encode_compoundres() relies on the NFS
dispatcher to pass in the buffer location of the COMPOUND status.
Instead, save that buffer location in struct nfsd4_compoundres.

The compound tag follows immediately after.

Signed-off-by: Chuck Lever <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs4proc.c

···
 	__be32		status;
 
 	resp->xdr = &rqstp->rq_res_stream;
+	resp->statusp = resp->xdr->p;
 
 	/* reserve space for: NFS status code */
 	xdr_reserve_space(resp->xdr, XDR_UNIT);
 
-	resp->tagp = resp->xdr->p;
 	/* reserve space for: taglen, tag, and opcnt */
 	xdr_reserve_space(resp->xdr, XDR_UNIT * 2 + args->taglen);
 	resp->taglen = args->taglen;

fs/nfsd/nfs4xdr.c

···
 	WARN_ON_ONCE(buf->len != buf->head[0].iov_len + buf->page_len +
 				 buf->tail[0].iov_len);
 
-	*p = resp->cstate.status;
+	/*
+	 * Send buffer space for the following items is reserved
+	 * at the top of nfsd4_proc_compound().
+	 */
+	p = resp->statusp;
+
+	*p++ = resp->cstate.status;
 
 	rqstp->rq_next_page = resp->xdr->page_ptr + 1;
 
-	p = resp->tagp;
 	*p++ = htonl(resp->taglen);
 	memcpy(p, resp->tag, resp->taglen);
 	p += XDR_QUADLEN(resp->taglen);

fs/nfsd/xdr4.h

···
 	struct xdr_stream		*xdr;
 	struct svc_rqst *		rqstp;
 
+	__be32				*statusp;
 	u32				taglen;
 	char *				tag;
 	u32				opcnt;
-	__be32 *			tagp; /* tag, opcount encode location */
+
 	struct nfsd4_compound_state	cstate;
 };