bgpd: Fix silly mistake in bgp_attr_flag_invalid

* bgp_attr.c: (bgp_attr_flag_invalid) flags is meant to be masked
  off with the mask variable...
diff --git a/bgpd/bgp_attr.c b/bgpd/bgp_attr.c
index d204cec..6670416 100644
--- a/bgpd/bgp_attr.c
+++ b/bgpd/bgp_attr.c
@@ -892,7 +892,7 @@
       && CHECK_FLAG (flags, BGP_ATTR_FLAG_TRANS))
     SET_FLAG (mask, BGP_ATTR_FLAG_PARTIAL);
   
-  if ((flags & ~attr_flags_values[attr_code])
+  if ((flags & ~mask)
       == attr_flags_values[attr_code])
     return 0;