Issue adding IPv6 to namespace node when default IPv6 GW already learnt from network generated RA

[haklat43]

ip -6 route add does not work if there is already default IPv6 GW in the namespace. It might happen occasionally, in the process of adding IPv6 to a TRex namespace node, that we receive a Router Advertisement from the network. When this RA is taken as default route in the namespace. A little later when TRex code adds IPv6 default for the namespace by "ip -6 route add default" this will fail because there is already ipv6 default configured by the network generated (and by TRex received) RA message.

Better option is to use replace rather than add. I.e. change:
ip -6 route add default

to:

ip -6 route replace default

With replace instead of add the process of creating namespaces with IPv6 is more robust.
File to patch:

src/stx/common/trex_stack_linux_based.cpp