bgpd: Regularise BGP NLRI sanity checks a bit

* bgp_route.h: (bgp_nlri_sanity_check) The bulk of the args are equivalent
  to a (struct bgp_nlri), consolidate.
* bgp_route.c: (bgp_nlri_sanity_check) Make this a frontend for all afi/safis.
  Including SAFI_MPLS_LABELED_VPN.
  (bgp_nlri_sanity_check_ip) Regular IP NLRI sanity check based on the
  existing code, and adjusted for (struct bgp_nlri *) arg.
* bgp_attr.c: (bgp_mp_reach_parse) Adjust for passing (struct bgp_nlri *)
  to bgp_nlri_sanity_check.
  Get rid of special-casing to not sanity check VPN.
  (bgp_mp_unreach_parse) Ditto.

* bgp_mplsvpn.c: Use the same VPN parsing code for both the sanity
  check and the actual parse.

  (bgp_nlri_parse_vpn) renamed to bgp_nlri_parse_vpn_body and made
  internal.

  (bgp_nlri_parse_vpn_body) Added (bool) argument to control whether it
  is sanity checking or whether it should update routing state for each
  NLRI.  Send a NOTIFY and reset the session, if there's a parsing
  error, as bgp_nlri_sanity_check_ip does, and as is required by the
  RFC.

  (bgp_nlri_parse_vpn) now a wrapper to call _body with update.

  (bgp_nlri_sanity_check_vpn) wrapper to call parser without
  updating.

* bgp_mplsvpn.h: (bgp_nlri_sanity_check_vpn) export for
  bgp_nlri_sanity_check.

* bgp_packet.c: (bgp_update_receive) Adjust for bgp_nlri_sanity_check
  argument changes.

* test/bgp_mp_attr_test.c: Extend to also test the NLRI parsing functions,
  if the initial MP-attr parsing has succeeded.  Fix the NLRI in the
  VPN cases.  Add further VPN tests.

* tests/bgpd.tests/testbgpmpattr.exp: Add the new test cases.

This commit a joint effort of:

Lou Berger <lberger@labn.net>
Donald Sharp <sharpd@cumulusnetworks.com>
Paul Jakma <paul.jakma@hpe.com> / <paul@jakma.org>
diff --git a/bgpd/bgp_packet.c b/bgpd/bgp_packet.c
index b8a38fa..d40c24c 100644
--- a/bgpd/bgp_packet.c
+++ b/bgpd/bgp_packet.c
@@ -1660,17 +1660,17 @@
   /* Unfeasible Route packet format check. */
   if (withdraw_len > 0)
     {
-      ret = bgp_nlri_sanity_check (peer, AFI_IP, SAFI_UNICAST, stream_pnt (s), withdraw_len);
+      withdraw.afi = AFI_IP;
+      withdraw.safi = SAFI_UNICAST;
+      withdraw.nlri = stream_pnt (s);
+      withdraw.length = withdraw_len;
+      ret = bgp_nlri_sanity_check (peer, &withdraw);
       if (ret < 0)
 	return -1;
 
       if (BGP_DEBUG (packet, PACKET_RECV))
 	zlog_debug ("%s [Update:RECV] Unfeasible NLRI received", peer->host);
 
-      withdraw.afi = AFI_IP;
-      withdraw.safi = SAFI_UNICAST;
-      withdraw.nlri = stream_pnt (s);
-      withdraw.length = withdraw_len;
       stream_forward_getp (s, withdraw_len);
     }
   
@@ -1751,8 +1751,14 @@
 
   if (update_len)
     {
+      /* Set NLRI portion to structure. */
+      update.afi = AFI_IP;
+      update.safi = SAFI_UNICAST;
+      update.nlri = stream_pnt (s);
+      update.length = update_len;
+      
       /* Check NLRI packet format and prefix length. */
-      ret = bgp_nlri_sanity_check (peer, AFI_IP, SAFI_UNICAST, stream_pnt (s), update_len);
+      ret = bgp_nlri_sanity_check (peer, &update);
       if (ret < 0)
         {
           bgp_attr_unintern_sub (&attr);
@@ -1760,11 +1766,6 @@
 	  return -1;
 	}
 
-      /* Set NLRI portion to structure. */
-      update.afi = AFI_IP;
-      update.safi = SAFI_UNICAST;
-      update.nlri = stream_pnt (s);
-      update.length = update_len;
       stream_forward_getp (s, update_len);
     }