bgpd: Rejiggle exported nht function names and consolidate some code

* bgp_nht.h: Tweak the API a bit to simplify and make names a bit clearer on
  function. Remove AFI argument, it's implied in both bgp_infos and peers.

  (bgp_find_nexthop) this doesn't so much find a bnc, as check the bnc
  for the given bgp_info is valid. Rename to (bgp_nexthop_check).

  (bgp_find_or_add_nexthop) This ensures a bnc exists, so call it
  (bgp_ensure_nexthop).

  (bgp_unlink_nexthop_by_peer) Remove via peer.

* bgp_nht.c: Adjust to above.
  (bgp_get_nexthop_rn) helper to get the rn.
  (bgp_find_nexthop) further helper to get the bnc for path or peer.
  (bgp_unlink_nexthop_check) helper to check whether a bnc should go.
  (bgp_ensure_nexthop) Use the helpers.

* bgp_{route,fsm}.c: s/bgp_find_or_add_nexthop/bgp_ensure_nexthop/
4 files changed