nfsd4: don't treat readlink like a zero-copy operation

There's no advantage to this zero-copy-style readlink encoding, and it
unnecessarily limits the kinds of compounds we can handle.  (In practice
I can't see why a client would want e.g. multiple readlink calls in a
comound, but it's probably a spec violation for us not to handle it.)
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4xdr.c

@@ -3160,8 +3160,9 @@ static __be32
 nfsd4_encode_readlink(struct nfsd4_compoundres *resp, __be32 nfserr, struct nfsd4_readlink *readlink)
 {
 	int maxcount;
+	__be32 wire_count;
+	int zero = 0;
 	struct xdr_stream *xdr = &resp->xdr;
-	char *page;
 	int length_offset = xdr->buf->len;
 	__be32 *p;
 
@@ -3171,26 +3172,19 @@ nfsd4_encode_readlink(struct nfsd4_compoundres *resp, __be32 nfserr, struct nfsd
 	p = xdr_reserve_space(xdr, 4);
 	if (!p)
 		return nfserr_resource;
-
-	if (resp->xdr.buf->page_len)
-		return nfserr_resource;
-	if (!*resp->rqstp->rq_next_page)
-		return nfserr_resource;
-
-	page = page_address(*(resp->rqstp->rq_next_page++));
-
 	maxcount = PAGE_SIZE;
 
-	if (xdr->end - xdr->p < 1)
+	p = xdr_reserve_space(xdr, maxcount);
+	if (!p)
 		return nfserr_resource;
-
 	/*
 	 * XXX: By default, the ->readlink() VFS op will truncate symlinks
 	 * if they would overflow the buffer.  Is this kosher in NFSv4?  If
 	 * not, one easy fix is: if ->readlink() precisely fills the buffer,
 	 * assume that truncation occurred, and return NFS4ERR_RESOURCE.
 	 */
-	nfserr = nfsd_readlink(readlink->rl_rqstp, readlink->rl_fhp, page, &maxcount);
+	nfserr = nfsd_readlink(readlink->rl_rqstp, readlink->rl_fhp,
+						(char *)p, &maxcount);
 	if (nfserr == nfserr_isdir)
 		nfserr = nfserr_inval;
 	if (nfserr) {
@@ -3198,24 +3192,12 @@ nfsd4_encode_readlink(struct nfsd4_compoundres *resp, __be32 nfserr, struct nfsd
 		return nfserr;
 	}
 
-	WRITE32(maxcount);
-	resp->xdr.buf->head[0].iov_len = (char *)p
-				- (char *)resp->xdr.buf->head[0].iov_base;
-	resp->xdr.buf->page_len = maxcount;
-	xdr->buf->len += maxcount;
-	xdr->page_ptr += 1;
-	xdr->buf->buflen -= PAGE_SIZE;
-	xdr->iov = xdr->buf->tail;
-
-	/* Use rest of head for padding and remaining ops: */
-	resp->xdr.buf->tail[0].iov_base = p;
-	resp->xdr.buf->tail[0].iov_len = 0;
-	if (maxcount&3) {
-		p = xdr_reserve_space(xdr, 4);
-		WRITE32(0);
-		resp->xdr.buf->tail[0].iov_base += maxcount&3;
-		resp->xdr.buf->tail[0].iov_len = 4 - (maxcount&3);
-	}
+	wire_count = htonl(maxcount);
+	write_bytes_to_xdr_buf(xdr->buf, length_offset, &wire_count, 4);
+	xdr_truncate_encode(xdr, length_offset + 4 + maxcount);
+	if (maxcount & 3)
+		write_bytes_to_xdr_buf(xdr->buf, length_offset + 4 + maxcount,
+						&zero, 4 - (maxcount&3));
 	return 0;
 }