selftests: drv-net: fix merge conflicts resolution

After the recent merge between net-next and net, I got some conflicts on
my side because the merge resolution was different from Stephen's one
[1] I applied on my side in the MPTCP tree.

It looks like the code that is now in net-next is using the old way to
retrieve the local and remote addresses. This patch is now using the new
way, like what was in Stephen's email [1].

Also, in get_interface_info(), there were no conflicts in this area,
because that was new code from 'net', but a small adaptation was needed
there as well to get the remote address.

Fixes: 941defce ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
Link: https://lore.kernel.org/20250311115758.17a1d414@canb.auug.org.au

 [1]
Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250314-net-next-drv-net-ping-fix-merge-v1-1-0d5c19daf707@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>