selftests: net: test for lwtunnel dst ref loops

As recently specified by commit 0ea09cbf ("docs: netdev: add a note
on selftest posting") in net-next, the selftest is therefore shipped in
this series. However, this selftest does not really test this series. It
needs this series to avoid crashing the kernel. What it really tests,
thanks to kmemleak, is what was fixed by the following commits:
- commit c71a1929 ("net: ipv6: fix dst refleaks in rpl, seg6 and
ioam6 lwtunnels")
- commit 92191dd1 ("net: ipv6: fix dst ref loops in rpl, seg6 and
ioam6 lwtunnels")
- commit c64a0727 ("net: ipv6: fix dst ref loop on input in seg6
lwt")
- commit 13e55fba ("net: ipv6: fix dst ref loop on input in rpl
lwt")
- commit 0e7633d7 ("net: ipv6: fix dst ref loop in ila lwtunnel")
- commit 5da15a9c ("net: ipv6: fix missing dst ref drop in ila
lwtunnel")

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Link: https://patch.msgid.link/20250314120048.12569-4-justin.iurman@uliege.be


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