xdrgen: Fix the variable-length opaque field decoder template

Ensure that variable-length opaques are decoded into the named
field, and do not overwrite the structure itself.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: chucklever

tools/net/sunrpc/xdrgen/templates/C/struct/decoder/variable_length_opaque.j2

@@ -2,5 +2,5 @@
 {% if annotate %}
 	/* member {{ name }} (variable-length opaque) */
 {% endif %}
-	if (!xdrgen_decode_opaque(xdr, (opaque *)ptr, {{ maxsize }}))
+	if (!xdrgen_decode_opaque(xdr, &ptr->{{ name }}, {{ maxsize }}))
 		return false;