bgpd: Blackhole nexthops are not reachable

bgpd: Blackhole nexthops are not reachable

When bgp registers for a nexthop that is not reachable due
to the nexthop pointing to a blackhole, bgp is never going
to be able to reach it when attempting to open a connection.

Broken behavior:

<show bgp nexthop>
 192.168.161.204 valid [IGP metric 0], #paths 0, peer 192.168.161.204
  blackhole
  Last update: Thu Feb 11 09:46:10 2021

eva# show bgp ipv4 uni summ fail
BGP router identifier 10.10.3.11, local AS number 3235 vrf-id 0
BGP table version 40
RIB entries 78, using 14 KiB of memory
Peers 2, using 54 KiB of memory

Neighbor             EstdCnt DropCnt ResetTime Reason
192.168.161.204            0       0     never Waiting for peer OPEN

The log file fills up with this type of message:
2021-02-09T18:53:11.653433+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument
2021-02-09T18:53:21.654005+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument
2021-02-09T18:53:31.654381+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument
2021-02-09T18:53:41.654729+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument
2021-02-09T18:53:51.655147+00:00 nq-sjc6c-cor-01 bgpd[6548]: can't connect to 24.51.27.241 fd 26 : Invalid argument

As that the connect to a blackhole is correctly rejected by the kernel

Fixed behavior:

eva# show bgp ipv4 uni summ
BGP router identifier 10.10.3.11, local AS number 3235 vrf-id 0
BGP table version 40
RIB entries 78, using 14 KiB of memory
Peers 2, using 54 KiB of memory
Neighbor             V         AS   MsgRcvd   MsgSent   TblVer  InQ OutQ  Up/Down State/PfxRcd   PfxSnt Desc
annie(192.168.161.2) 4      64539    126264        39        0    0    0 00:01:36           38       40 N/A
192.168.161.178      4          0         0         0        0    0    0    never       Active        0 N/A
Total number of neighbors 2
eva# show bgp ipv4 uni summ fail
BGP router identifier 10.10.3.11, local AS number 3235 vrf-id 0
BGP table version 40
RIB entries 78, using 14 KiB of memory
Peers 2, using 54 KiB of memory
Neighbor             EstdCnt DropCnt ResetTime Reason
192.168.161.178            0       0     never Waiting for NHT
Total number of neighbors 2
eva# show bgp nexthop
Current BGP nexthop cache:
 192.168.161.2 valid [IGP metric 0], #paths 38, peer 192.168.161.2
  if enp39s0
  Last update: Thu Feb 11 09:52:05 2021
 192.168.161.131 valid [IGP metric 0], #paths 0, peer 192.168.161.131
  if enp39s0
  Last update: Thu Feb 11 09:52:05 2021
 192.168.161.178 invalid, #paths 0, peer 192.168.161.178
  Must be Connected
  Last update: Thu Feb 11 09:53:37 2021
eva#

Signed-off-by: Donald Sharp <sharpd@nvidia.com>