selftests: udpgso: Pull up network setup into shell script

selftests: udpgso: Pull up network setup into shell script

udpgso regression test configures routing and device MTU directly through
uAPI (Netlink, ioctl) to do its job. While there is nothing wrong with it,
it takes more effort than doing it from shell.

Looking forward, we would like to extend the udpgso regression tests to
cover the EIO corner case [1], once it gets addressed. That will require a
dummy device and device feature manipulation to set it up. Which means more
Netlink code.

So, in preparation, pull out network configuration into the shell script
part of the test, so it is easily extendable in the future.

Also, because it now easy to setup routing, add a second local IPv6
address. Because the second address is not managed by the kernel, we can
"replace" the corresponding local route with a reduced-MTU one. This
unblocks the disabled "ipv6 connected" test case. Add a similar setup for
IPv4 for symmetry.

[1] https://lore.kernel.org/netdev/87jzqsld6q.fsf@cloudflare.com/

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/r/20240207-jakub-krn-635-v3-1-3dfa3da8a7d3@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>