zebra: Create Singleton nhg's without weights

zebra: Create Singleton nhg's without weights

Currently FRR when it has two nexthop groups:
A
  nexthop 1 weight 5
  nexthop 2 weight 6
  nexthop 3 weight 7
B
  nexthop 1 weight 3
  nexthop 2 weight 4
  nexthop 3 weight 5

We end up with 5 singleton nexthops and two groups:

ID: 181818168 (sharp)
     RefCnt: 1
     Uptime: 00:04:52
     VRF: default
     Valid, Installed
     Depends: (69) (70) (71)
           via 192.168.119.1, enp13s0 (vrf default), weight 182
           via 192.168.119.2, enp13s0 (vrf default), weight 218
           via 192.168.119.3, enp13s0 (vrf default), weight 255
ID: 181818169 (sharp)
     RefCnt: 1
     Uptime: 00:02:08
     VRF: default
     Valid, Installed
     Depends: (71) (127) (128)
           via 192.168.119.1, enp13s0 (vrf default), weight 127
           via 192.168.119.2, enp13s0 (vrf default), weight 170
           via 192.168.119.3, enp13s0 (vrf default), weight 255

id 69 via 192.168.119.1 dev enp13s0 scope link proto 194
id 70 via 192.168.119.2 dev enp13s0 scope link proto 194
id 71 via 192.168.119.3 dev enp13s0 scope link proto 194
id 127 via 192.168.119.1 dev enp13s0 scope link proto 194
id 128 via 192.168.119.2 dev enp13s0 scope link proto 194
id 181818168 group 69,182/70,218/71,255 proto 194
id 181818169 group 71,255/127,127/128,170 proto 194

This is not a desirable state to be in.  If you have a
link flapping in the network and weights are changing
rapidly you end up with a large number of singleton
nexthops that are being used by the nexthop groups.
This fills up asic space and clutters the table.
Additionally singleton nexthops cannot have any weight
and the fact that you attempt to create a singleton
nexthop with different weights means nothing to the
linux kernel( or any asic dplane ).  Let's modify
the code to always create the singleton nexthops
without a weight and then just creating the
NHG's that use the singletons with the appropriate
weight.

ID: 181818168 (sharp)
     RefCnt: 1
     Uptime: 00:00:32
     VRF: default
     Valid, Installed
     Depends: (22) (24) (28)
           via 192.168.119.1, enp13s0 (vrf default), weight 182
           via 192.168.119.2, enp13s0 (vrf default), weight 218
           via 192.168.119.3, enp13s0 (vrf default), weight 255
ID: 181818169 (sharp)
     RefCnt: 1
     Uptime: 00:00:14
     VRF: default
     Valid, Installed
     Depends: (22) (24) (28)
           via 192.168.119.1, enp13s0 (vrf default), weight 153
           via 192.168.119.2, enp13s0 (vrf default), weight 204
           via 192.168.119.3, enp13s0 (vrf default), weight 255

id 22 via 192.168.119.1 dev enp13s0 scope link proto 194
id 24 via 192.168.119.2 dev enp13s0 scope link proto 194
id 28 via 192.168.119.3 dev enp13s0 scope link proto 194
id 181818168 group 22,182/24,218/28,255 proto 194
id 181818169 group 22,153/24,204/28,255 proto 194

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