nfsd4: fix buflen calculation after read encoding

We don't necessarily want to assume that the buflen is the same
as the number of bytes available in the pages.  We may have some reason
to set it to something less (for example, later patches will use a
smaller buflen to enforce session limits).

So, calculate the buflen relative to the previous buflen instead of
recalculating it from scratch.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4xdr.c

@@ -3053,6 +3053,7 @@ nfsd4_encode_read(struct nfsd4_compoundres *resp, __be32 nfserr,
 	unsigned long maxcount; 
 	struct xdr_stream *xdr = &resp->xdr;
 	int starting_len = xdr->buf->len;
+	int space_left;
 	long len;
 	__be32 *p;
 
@@ -3117,7 +3118,6 @@ nfsd4_encode_read(struct nfsd4_compoundres *resp, __be32 nfserr,
 	resp->xdr.buf->page_len = maxcount;
 	xdr->buf->len += maxcount;
 	xdr->page_ptr += v;
-	xdr->buf->buflen = maxcount + PAGE_SIZE - 2 * RPC_MAX_AUTH_SIZE;
 	xdr->iov = xdr->buf->tail;
 
 	/* Use rest of head for padding and remaining ops: */
@@ -3130,6 +3130,12 @@ nfsd4_encode_read(struct nfsd4_compoundres *resp, __be32 nfserr,
 		resp->xdr.buf->tail[0].iov_len = 4 - (maxcount&3);
 		xdr->buf->len -= (maxcount&3);
 	}
+
+	space_left = min_t(int, (void *)xdr->end - (void *)xdr->p,
+				xdr->buf->buflen - xdr->buf->len);
+	xdr->buf->buflen = xdr->buf->len + space_left;
+	xdr->end = (__be32 *)((void *)xdr->end + space_left);
+
 	return 0;
 }