SUNRPC: fix decoding of optional gss-proxy xdr fields

The current code works, but sort of by accident: it obviously didn't
intend the error return to be interpreted as "true".
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/auth_gss/gss_rpc_xdr.c

@@ -21,16 +21,6 @@
 #include <linux/sunrpc/svcauth.h>
 #include "gss_rpc_xdr.h"
 
-static bool gssx_check_pointer(struct xdr_stream *xdr)
-{
-	__be32 *p;
-
-	p = xdr_reserve_space(xdr, 4);
-	if (unlikely(p == NULL))
-		return -ENOSPC;
-	return *p?true:false;
-}
-
 static int gssx_enc_bool(struct xdr_stream *xdr, int v)
 {
 	__be32 *p;
@@ -802,6 +792,7 @@ int gssx_dec_accept_sec_context(struct rpc_rqst *rqstp,
 				struct xdr_stream *xdr,
 				struct gssx_res_accept_sec_context *res)
 {
+	u32 value_follows;
 	int err;
 
 	/* res->status */
@@ -810,7 +801,10 @@ int gssx_dec_accept_sec_context(struct rpc_rqst *rqstp,
 		return err;
 
 	/* res->context_handle */
-	if (gssx_check_pointer(xdr)) {
+	err = gssx_dec_bool(xdr, &value_follows);
+	if (err)
+		return err;
+	if (value_follows) {
 		err = gssx_dec_ctx(xdr, res->context_handle);
 		if (err)
 			return err;
@@ -819,7 +813,10 @@ int gssx_dec_accept_sec_context(struct rpc_rqst *rqstp,
 	}
 
 	/* res->output_token */
-	if (gssx_check_pointer(xdr)) {
+	err = gssx_dec_bool(xdr, &value_follows);
+	if (err)
+		return err;
+	if (value_follows) {
 		err = gssx_dec_buffer(xdr, res->output_token);
 		if (err)
 			return err;
@@ -828,7 +825,10 @@ int gssx_dec_accept_sec_context(struct rpc_rqst *rqstp,
 	}
 
 	/* res->delegated_cred_handle */
-	if (gssx_check_pointer(xdr)) {
+	err = gssx_dec_bool(xdr, &value_follows);
+	if (err)
+		return err;
+	if (value_follows) {
 		/* we do not support upcall servers sending this data. */
 		return -EINVAL;
 	}