bgpd: don't send NOTIFY twice for malformed attrs

Most of the attribute parsing functions were already sending a notify,
let's clean up the code to make it happen only once.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/bgpd/bgp_attr.h b/bgpd/bgp_attr.h
index cdd5467..cb401e7 100644
--- a/bgpd/bgp_attr.h
+++ b/bgpd/bgp_attr.h
@@ -136,6 +136,9 @@
  BGP_ATTR_PARSE_PROCEED = 0,
  BGP_ATTR_PARSE_ERROR = -1,
  BGP_ATTR_PARSE_WITHDRAW = -2,
+
+ /* only used internally, send notify + convert to BGP_ATTR_PARSE_ERROR */
+ BGP_ATTR_PARSE_ERROR_NOTIFYPLS = -3,
 } bgp_attr_parse_ret_t;
 
 /* Prototypes. */