bgpd: fix regression in improved attr flag checks

Commit 2febf323411c1aed9d7694898f852ce2ef36a7e5 assumed every flag
bit except optional/transitive/partial unset, which at times could
not be true for "extended length" bit.

* bgp_attr.c
  * bgp_attr_origin(): exclude BGP_ATTR_FLAG_EXTLEN from comparison
  * bgp_attr_nexthop(): idem
  * bgp_attr_med(): idem
  * bgp_attr_local_pref(): idem
  * bgp_attr_atomic(): idem
1 file changed